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C – Language History

• The C programming language is a structure oriented programming language, developed at Bell

Laboratories in 1972 by Dennis Ritchie

• C programming language features were derived from an earlier language called “B” (Basic Combined

Programming Language – BCPL)

• C language was invented for implementing UNIX operating system

• In 1978, Dennis Ritchie and Brian Kernighan published the first edition “The C Programming Language” and

commonly known as K&R C

• In 1983, the American National Standards Institute (ANSI) established a committee to provide a modern,

comprehensive definition of C. The resulting definition, the ANSI standard, or “ANSI C”, was completed late

1988.

C programming language standards:

• C89/C90 standard – First standardized specification for C language was developed by the American

National Standards Institute in 1989. C89 and C90 standards refer to the same programming language.

• C99 standard – Next revision was published in 1999 that introduced new features like advanced data types

and other changes.

C11 and Embedded C language:

• C11 standard adds new features to C programming language and library like type generic macros,

anonymous structures, improved Unicode support, atomic operations, multi-threading and bounds-checked

functions. It also makes some portions of the existing C99 library optional and improves compatibility with

C++.

• Embedded C includes features not available in C like fixed-point arithmetic, named address spaces, and

basic I/O hardware addressing.

• Operating systems, C compiler and all UNIX application programs are written in C language

• It is also called as procedure oriented programming language. The C language is reliable, simple and easy

to use. C has been coded in assembly language.

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Features of C programming language:

• Reliability

• Portability

• Flexibility

• Interactivity

• Modularity

• Efficiency and Effectiveness

Uses of C programming language:

The C programming language is used for developing system applications that forms a major portion of operating

systems such as Windows, UNIX and Linux. Below are some examples of C being used.

• Database systems

• Graphics packages

• Word processors

• Spreadsheets

• Operating system development

• Compilers and Assemblers

• Network drivers

• Interpreters

Which level is C language belonging to?

S.no High Level Middle Level Low Level 1 High level languages

provide almost everything that the programmer might need to do as already built into the language

Middle level languages don’t provide all the built-in functions found in high level languages, but provides all building blocks that we need to produce the result we want

Low level languages provides nothing other than access to the machines basic instruction set

2 Examples: Java, Python

C, C++ Assembler

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The C language is a structured language

S.no Structure oriented Object oriented Non structure 1 In this type of language,

large programs are divided into small programs called functions

In this type of language, programs are divided into objects

There is no specific structure for programming this language

2 Prime focus is on functions and procedures that operate on the data

Prime focus is in the data that is being operated and not on the functions or procedures

N/A

3 Data moves freely around the systems from one function to another

Data is hidden and cannot be accessed by external functions

N/A

4 Program structure follows “Top Down Approach”

Program structure follows “Bottom UP Approach”

N/A

5 Examples: C, Pascal, ALGOL and Modula-2

C++, JAVA and C# (C sharp)

BASIC, COBOL, FORTRAN

Key points to remember in C language:

1. The C language is structured, middle level programming language developed by Dennis Ritchie 2. Operating system programs such as Windows, Unix, Linux are written in C language 3. C89/C90 and C99 are two standardized editions of C language 4. C has been written in assembly language

C language tutorial reference E-books & research papers:

• [ANSI 89] American National Standards Institute., American National Standard for Information Programming

Language C, X3 159-1989

• [Kernighan 78] B. W. Kernighan and D. M. Ritchie, The C Programming Language, Prentice-

Hall: Englewood Cliffs, NJ, 1978. Second edition, 1988.

• [Thinking 90] C* Programming Guide, Thinking Machines Corp.: Cambridge Mass., 1990.

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C – Basic Program

We are going to learn a simple “Hello World” C program in this section. Also, all the below topics are explained in this section which are the basics of a C program.

1. C basic program with output and explanation

2. Steps to write C programs and get the output

3. Creation, Compilation and Execution of a C program

1. How to install C compiler and IDE

4. Basic structure of a C program

Basic commands in C programming to write basic C Program:

Below are few commands and syntax used in C programming to write a simple C program. Let’s see all the sections

of a simple C program line by line.

S.no Command Explanation

1 #include <stdio.h> This is a preprocessor command that includes standard input output header file(stdio.h) from the C library before compiling a C program

2 int main() This is the main function from where execution of any C program begins.

3 { This indicates the beginning of the main function.

4 /*_some_comments_*/ whatever is given inside the command “/* */” in any C program, won’t be considered for compilation and execution.

5 printf(“Hello_World! “); printf command prints the output onto the screen.

6 getch(); This command waits for any character input from keyboard.

7 return 0; This command terminates C program (main function) and returns 0.

8 } This indicates the end of the main function.

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1. C Basic Program:

#include <stdio.h> int main() { /* Our first simple C basic program */ printf(“Hello World! “); getch(); return 0; } .

Output:

Hello World! .

2.Steps to write C programs and get the output:

Below are the steps to be followed for any C program to create and get the output. This is common to all C program and there is no exception whether its a very small C program or very large C program.

3. Creation, Compilation and Execution of a C program:

Prerequisite:

• If you want to create, compile and execute C programs by your own, you have to install C compiler in your machine. Then, you can start to execute your own C programs in your machine.

• You can refer below link for how to install C compiler and compile and execute C programs in your machine. • Once C compiler is installed in your machine, you can create, compile and execute C programs as shown in

below link.

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4. Basic structure of C program:

Structure of C program is defined by set of rules called protocol, to be followed by programmer while writing C program. All C programs are having sections/parts which are mentioned below.

1. Documentation section 2. Link Section 3. Definition Section 4. Global declaration section 5. Function prototype declaration section 6. Main function 7. User defined function definition section

Example C program to compare all the sections:

You can compare all the sections of a C program with the below C program.

/* C basic structure program Documentation sectionAuthor: fresh2refresh.com Date : 01/01/2012 */#include <stdio.h> /* Link section */ int total = 0; /* Global declaration and definition section */ int sum (int, int); /* Function declaration section */ int main () /* Main function */ { printf (“This is a C basic program \n”); total = sum (1, 1); printf (“Sum of two numbers : %d \n”, total); return 0; } int sum (int a, int b) /* User defined function */ { /* definition section */ return a + b; } .

Output:

This is a C basic program Sum of two numbers : 2 .

Description for each section of a C program:

• Let us see about each section of a C basic program in detail below. • Please note that a C program mayn’t have all below mentioned sections except main function and link

sections. • Also, a C program structure mayn’t be in below mentioned order.

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S.No Sections Description

1 Documentation section

We can give comments about the program, creation or modified date, author name etc in this section. The characters or words or anything which are given between “/*” and “*/”, won’t be considered by C compiler for compilation process.These will be ignored by C compiler during compilation. Example : /* comment line1 comment line2 comment 3 */

2 Link Section Header files that are required to execute a C program are included in this section

3 Definition Section In this section, variables are defined and values are set to these variables.

4 Global declaration section

Global variables are defined in this section. When a variable is to be used throughout the program, can be defined in this section.

5 Function prototype declaration section

Function prototype gives many information about a function like return type, parameter names used inside the function.

6 Main function Every C program is started from main function and this function contains two major sections called declaration section and executable section.

7 User defined function section

User can define their own functions in this section which perform particular task as per the user requirement.

C programming tutorial reference E-books & research papers:

• [ANSI 89] American National Standards Institute., American National Standard for Information Programming

Language C, X3 159-1989

• [Kernighan 78] B. W. Kernighan and D. M. Ritchie, The C Programming Language, Prentice-

Hall: Englewood Cliffs, NJ, 1978. Second edition, 1988.

• [Thinking 90] C* Programming Guide, Thinking Machines Corp.: Cambridge Mass., 1990.

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C – printf and scanf

• printf() and scanf() functions are inbuilt library functions in C which are available in C library by default. These functions are declared and related macros are defined in “stdio.h” which is a header file.

• We have to include “stdio.h” file as shown in below C program to make use of these printf() and scanf() library functions.

1. C printf() function:

• printf() function is used to print the “character, string, float, integer, octal and hexadecimal values” onto the output screen.

• We use printf() function with %d format specifier to display the value of an integer variable. • Similarly %c is used to display character, %f for float variable, %s for string variable, %lf for double

and %x for hexadecimal variable. • To generate a newline,we use “\n” in C printf() statement.

Note:

• C language is case sensitive. For example, printf() and scanf() are different from Printf() and Scanf(). All characters in printf() and scanf() functions must be in lower case.

Example program for C printf() function:

#include <stdio.h> int main() { char ch = ‘A’; char str[20] = “fresh2refresh.com”; float flt = 10.234; int no = 150; double dbl = 20.123456; printf(“Character is %c \n”, ch); printf(“String is %s \n” , str); printf(“Float value is %f \n”, flt); printf(“Integer value is %d\n” , no); printf(“Double value is %lf \n”, dbl); printf(“Octal value is %o \n”, no); printf(“Hexadecimal value is %x \n”, no); return 0; } .

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Output:

Character is A String is fresh2refresh.com Float value is 10.234000 Integer value is 150 Double value is 20.123456 Octal value is 226 Hexadecimal value is 96 .

You can see the output with the same data which are placed within the double quotes of printf statement in the program except

• %d got replaced by value of an integer variable (no),

• %c got replaced by value of a character variable (ch),

• %f got replaced by value of a float variable (flt),

• %lf got replaced by value of a double variable (dbl),

• %s got replaced by value of a string variable (str),

• %o got replaced by a octal value corresponding to integer variable (no),

• %x got replaced by a hexadecimal value corresponding to integer variable

• \n got replaced by a newline.

2. C scanf() function:

• scanf() function is used to read character, string, numeric data from keyboard • Consider below example program where user enters a character. This value is assigned to the variable “ch”

and then displayed.

• Then, user enters a string and this value is assigned to the variable ”str” and then displayed.

Example program for printf() and scanf() functions in C:

#include <stdio.h> int main() { char ch; char str[100]; printf(“Enter any character \n”); scanf(“%c”, &ch); printf(“Entered character is %c \n”, ch); printf(“Enter any string ( upto 100 character ) \n”); scanf(“%s”, &str); printf(“Entered string is %s \n”, str); } .

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Output:

Enter any character a Entered character is a Enter any string ( upto 100 character ) hai Entered string is hai .

• The format specifier %d is used in scanf() statement. So that, the value entered is received as an integer and %s for string.

• Ampersand is used before variable name “ch” in scanf() statement as &ch. • It is just like in a pointer which is used to point to the variable. For more information about how pointer

works, please click here.

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C – Data Types

• C data types are defined as the data storage format that a variable can store a data to perform a specific operation.

• Data types are used to define a variable before to use in a program. • Size of variable, constant and array are determined by data types.

C – data types:

There are four data types in C language. They are,

S.no Types Data Types 1 Basic data types int, char, float, double 2 Enumeration data type enum 3 Derived data type pointer, array, structure, union 4 Void data type void

1. Basic data types in C:

1.1. Integer data type:

• Integer data type allows a variable to store numeric values. • “int” keyword is used to refer integer data type. • The storage size of int data type is 2 or 4 or 8 byte. • It varies depend upon the processor in the CPU that we use. If we are using 16 bit processor, 2 byte (16

bit) of memory will be allocated for int data type. • Like wise, 4 byte (32 bit) of memory for 32 bit processor and 8 byte (64 bit) of memory for 64 bit processor is

allocated for int datatype. • int (2 byte) can store values from -32,768 to +32,767 • int (4 byte) can store values from -2,147,483,648 to +2,147,483,647. • If you want to use the integer value that crosses the above limit, you can go for “long int” and “long long int”

for which the limits are very high.

Note:

• We can’t store decimal values using int data type. • If we use int data type to store decimal values, decimal values will be truncated and we will get only whole

number. • In this case, float data type can be used to store decimal values in a variable.

1.2. Character data type:

• Character data type allows a variable to store only one character. • Storage size of character data type is 1. We can store only one character using character data type. • “char” keyword is used to refer character data type. • For example, ‘A’ can be stored using char datatype. You can’t store more than one character using char

data type. • Please refer C – Strings topic to know how to store more than one characters in a variable.

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1.3. Floating point data type:

Floating point data type consists of 2 types. They are,

1. float 2. double

1. float:

• Float data type allows a variable to store decimal values. • Storage size of float data type is 4. This also varies depend upon the processor in the CPU as “int” data

type. • We can use up-to 6 digits after decimal using float data type. • For example, 10.456789 can be stored in a variable using float data type.

2. double:

• Double data type is also same as float data type which allows up-to 10 digits after decimal. • The range for double datatype is from 1E–37 to 1E+37.

1.3.1. sizeof() function in C:

sizeof() function is used to find the memory space allocated for each C data types.

#include <stdio.h> #include <limits.h> int main() { int a; char b; float c; double d; printf(“Storage size for int data type:%d \n”,sizeof(a)); printf(“Storage size for char data type:%d \n”,sizeof(b)); printf(“Storage size for float data type:%d \n”,sizeof(c)); printf(“Storage size for double data type:%d\n”,sizeof(d)); return 0; } . Output:

Storage size for int data type:4 Storage size for char data type:1 Storage size for float data type:4 Storage size for double data type:8 .

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1.3.2. Modifiers in C:

• The amount of memory space to be allocated for a variable is derived by modifiers. • Modifiers are prefixed with basic data types to modify (either increase or decrease) the amount of storage

space allocated to a variable. • For example, storage space for int data type is 4 byte for 32 bit processor. We can increase the range by

using long int which is 8 byte. We can decrease the range by using short int which is 2 byte.

• There are 5 modifiers available in C language. They are,

1. short 2. long 3. signed 4. unsigned 5. long long

• Below table gives the detail about the storage size of each C basic data type in 16 bit processor. Please keep in mind that storage size and range for int and float datatype will vary depend on the CPU processor (8,16, 32 and 64 bit)

S.No C Data types storage Size Range 1 char 1 –127 to 127 2 int 2 –32,767 to 32,767 3 float 4 1E–37 to 1E+37 with six digits of precision 4 double 8 1E–37 to 1E+37 with ten digits of precision5 long double 10 1E–37 to 1E+37 with ten digits of precision6 long int 4 –2,147,483,647 to 2,147,483,647 7 short int 2 –32,767 to 32,767 8 unsigned short int 2 0 to 65,535 9 signed short int 2 –32,767 to 32,767

10 long long int 8 –(2power(63) –1) to 2(power)63 –1 11 signed long int 4 –2,147,483,647 to 2,147,483,647 12 unsigned long int 4 0 to 4,294,967,295 13 unsigned long long int 8 2(power)64 –1

2. Enumeration data type in C:

• Enumeration data type consists of named integer constants as a list. • It start with 0 (zero) by default and value is incremented by 1 for the sequential identifiers in the list.

• Enum syntax in C:

enum identifier [optional{ enumerator-list }];

• Enum example in C:

enum month { Jan, Feb, Mar }; or /* Jan, Feb and Mar variables will be assigned to 0, 1 and 2 respectively by default */ enum month { Jan = 1, Feb, Mar }; /* Feb and Mar variables will be assigned to 2 and 3 respectively by default */

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enum month { Jan = 20, Feb, Mar }; /* Jan is assigned to 20. Feb and Mar variables will be assigned to 21 and 22 respectively by default */

• The above enum functionality can also be implemented by “#define” preprocessor directive as given below. Above enum example is same as given below.

#define Jan 20; #define Feb 21; #define Mar 22;

C – enum example program:

#include <stdio.h> int main() { enum MONTH { Jan = 0, Feb, Mar }; enum MONTH month = Mar; if(month == 0) printf(“Value of Jan”); else if(month == 1) printf(“Month is Feb”); if(month == 2) printf(“Month is Mar”); } . Output:

Month is March .

3. Derived data type in C:

• Array, pointer, structure and union are called derived data type in C language. • To know more about derived data types, please visit “C – Array“ , “C – Pointer” , “C – Structure” and “C –

Union” topics in this tutorial.

4. Void data type in C:

• Void is an empty data type that has no value. • This can be used in functions and pointers. • Please visit “C – Function” topic to know how to use void data type in function with simple call by value and

call by reference example programs.

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C – Tokens and keywords

C tokens, Identifiers and Keywords are the basics in a C program. All are explained in this page with definition and simple example programs.

1. C tokens:

• C tokens are the basic buildings blocks in C language which are constructed together to write a C program. • Each and every smallest individual units in a C program are known as C tokens.

• C tokens are of six types. They are,

1. Keywords (eg: int, while), 2. Identifiers (eg: main, total), 3. Constants (eg: 10, 20), 4. Strings (eg: “total”, “hello”), 5. Special symbols (eg: (), {}), 6. Operators (eg: +, /,-,*)

C tokens example program:

int main() { int x, y, total; x = 10, y = 20; total = x + y; Printf (“Total = %d \n”, total); } .

where,

• main – identifier • {,}, (,) – delimiter • int – keyword • x, y, total – identifier • main, {, }, (, ), int, x, y, total – tokens

Do you know how to use C token in real time application programs? We have given simple real time application programs where C token is used. You can refer the below C programs to know how to use C token in real time program.

2. Identifiers in C language:

• Each program elements in a C program are given a name called identifiers. • Names given to identify Variables, functions and arrays are examples for identifiers. eg. x is a name given to

integer variable in above program.

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Rules for constructing identifier name in C:

1. First character should be an alphabet or underscore. 2. Succeeding characters might be digits or letter. 3. Punctuation and special characters aren’t allowed except underscore. 4. Identifiers should not be keywords.

3. Keywords in C language:

• Keywords are pre-defined words in a C compiler. • Each keyword is meant to perform a specific function in a C program. • Since keywords are referred names for compiler, they can’t be used as variable name.

C language supports 32 keywords which are given below.

auto double int struct const float short unsigned

break else long switch continue for signed void case enum register typedef default goto sizeof volatile char extern return union do if static while

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C – Constant

• C Constants are also like normal variables. But, only difference is, their values can not be modified by the program once they are defined.

• Constants refer to fixed values. They are also called as literals • Constants may be belonging to any of the data type.

Syntax: const data_type variable_name; (or) const data_type *variable_name;

Types of C constant:

1. Integer constants 2. Real or Floating point constants 3. Octal & Hexadecimal constants 4. Character constants 5. String constants 6. Backslash character constants

S.no Constant type data type Example 1 Integer constants int

unsigned intlong int long long int

53, 762, -478 etc 5000u, 1000U etc 483,647 2,147,483,680

2 Real or Floating point constants float doule

10.456789 600.123456789

3 Octal constant int 013 /* starts with 0 */4 Hexadecimal constant int 0×90 /* starts with 0x */5 character constants char ‘A’ , ‘B’, ‘C’ 6 string constants char “ABCD” , “Hai”

Rules for constructing C constant:

1. Integer Constants in C:

• An integer constant must have at least one digit. • It must not have a decimal point. • It can either be positive or negative. • No commas or blanks are allowed within an integer constant. • If no sign precedes an integer constant, it is assumed to be positive. • The allowable range for integer constants is -32768 to 32767.

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2. Real constants in C:

• A real constant must have at least one digit • It must have a decimal point • It could be either positive or negative • If no sign precedes an integer constant, it is assumed to be positive. • No commas or blanks are allowed within a real constant.

3. Character and string constants in C:

• A character constant is a single alphabet, a single digit or a single special symbol enclosed within single quotes.

• The maximum length of a character constant is 1 character. • String constants are enclosed within double quotes.

4. Backslash Character Constants in C:

• There are some characters which have special meaning in C language. • They should be preceded by backslash symbol to make use of special function of them. • Given below is the list of special characters and their purpose.

Backslash_character Meaning \b Backspace \f Form feed \n New line \r Carriage return \t Horizontal tab \” Double quote \’ Single quote \\ Backslash \v Vertical tab \a Alert or bell \? Question mark \N Octal constant (N is an octal constant)

\XN Hexadecimal constant (N – hex.dcml cnst)

How to use constants in a C program?

• We can define constants in a C program in the following ways.

1. By “const” keyword 2. By “#define” preprocessor directive

• Please note that when you try to change constant values after defining in C program, it will through error.

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1. Example program using const keyword in C:

#include <stdio.h> void main() { const int height = 100; /*int constant*/ const float number = 3.14; /*Real constant*/ const char letter = ‘A’; /*char constant*/ const char letter_sequence[10] = “ABC”; /*string constant*/ const char backslash_char = ‘\?’; /*special char cnst*/ printf(“value of height :%d \n”, height ); printf(“value of number : %f \n”, number ); printf(“value of letter : %c \n”, letter ); printf(“value of letter_sequence : %s \n”, letter_sequence); printf(“value of backslash_char : %c \n”, backslash_char); }

Output:

value of height : 100 value of number : 3.140000 value of letter : A value of letter_sequence : ABC value of backslash_char : ?

2. Example program using #define preprocessor directive in C:

#include <stdio.h> #define height 100 #define number 3.14 #define letter ‘A’ #define letter_sequence “ABC” #define backslash_char ‘\?’ void main() { printf(“value of height : %d \n”, height ); printf(“value of number : %f \n”, number ); printf(“value of letter : %c \n”, letter ); printf(“value of letter_sequence : %s \n”,letter_sequence); printf(“value of backslash_char : %c \n”,backslash_char); }

Output:


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C – Variable

C variable is a named location in a memory where a program can manipulate the data. This location is used to hold the value of the variable.

• The value of the C variable may get change in the program. • C variable might be belonging to any of the data type like int, float, char etc.

Rules for naming C variable:

1. Variable name must begin with letter or underscore. 2. Variables are case sensitive 3. They can be constructed with digits, letters. 4. No special symbols are allowed other than underscore. 5. sum, height, _value are some examples for variable name

Declaring & initializing C variable:

• Variables should be declared in the C program before to use. • Memory space is not allocated for a variable while declaration. It happens only on variable definition. • Variable initialization means assigning a value to the variable.

S.No Type Syntax Example 1 Variable

declaration data_type variable_name; int x, y, z; char flat, ch;

2 Variable initialization

data_type variable_name = value;

int x = 50, y = 30; char flag = ‘x’, ch=’l’;

There are three types of variables in C program They are,

1. Local variable 2. Global variable 3. Environment variable

1. Example program for local variable in C:

• The scope of local variables will be within the function only. • These variables are declared within the function and can’t be accessed outside the function. • In the below example, m and n variables are having scope within the main function only. These are not

visible to test function. • Like wise, a and b variables are having scope within the test function only. These are not visible to main

function.

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#include<stdio.h> void test(); int main() { int m = 22, n = 44; // m, n are local variables of main function /*m and n variables are having scope within this main function only. These are not visible to test funtion.*/ /* If you try to access a and b in this function, you will get ‘a’ undeclared and ‘b’ undeclared error */ printf(“\nvalues : m = %d and n = %d”, m, n); test(); } void test() { int a = 50, b = 80; // a, b are local variables of test function /*a and b variables are having scope within this test function only. These are not visible to main function.*/ /* If you try to access m and n in this function, you will get ‘m’ undeclared and ‘n’ undeclared error */printf(“\nvalues : a = %d and b = %d”, a, b); }

Output:

values : m = 22 and n = 44 values : a = 50 and b = 80

2. Example program for global variable in C:

• The scope of global variables will be throughout the program. These variables can be accessed from anywhere in the program.

• This variable is defined outside the main function. So that, this variable is visible to main function and all other sub functions.

#include<stdio.h> void test();int m = 22, n = 44; int a = 50, b = 80; int main() { printf(“All variables are accessed from main function”); printf(“\nvalues: m=%d:n=%d:a=%d:b=%d”, m,n,a,b); test(); } void test() { printf(“\n\nAll variables are accessed from” \ ” test function”); printf(“\nvalues: m=%d:n=%d:a=%d:b=%d”, m,n,a,b); }

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Output:

All variables are accessed from main function values : m = 22 : n = 44 : a = 50 : b = 80

All variables are accessed from test function values : m = 22 : n = 44 : a = 50 : b = 80

3. Environment variables in C:

• Environment variable is a variable that will be available for all C applications and C programs. • We can access these variables from anywhere in a C program without declaring and initializing in an

application or C program. • The inbuilt functions which are used to access, modify and set these environment variables are called

environment functions.

• There are 3 functions which are used to access, modify and assign an environment variable in C. They are,

1. setenv() 2. getenv() 3. putenv()

Example program for getenv() function in C:

This function gets the current value of the environment variable. Let us assume that environment variable DIR is assigned to “/usr/bin/test/”.

#include <stdio.h> #include <stdlib.h> int main() { printf(“Directory = %s\n”,getenv(“DIR”)); return 0; }

Output:

/usr/bin/test/

Example program for setenv() function in C:

This function sets the value for environment variable. Let us assume that environment variable “FILE” is to be assigned “/usr/bin/example.c”

#include <stdio.h> #include <stdlib.h> int main() { setenv(“FILE”, “/usr/bin/example.c”,50); printf(“File = %s\n”, getenv(“FILE”)); return 0; }

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Output:

File = /usr/bin/example.c

Example program for putenv() function in C:

This function modifies the value for environment variable. Below example program shows that how to modify an existing environment variable value.

#include <stdio.h> #include <stdlib.h> int main() { setenv(“DIR”, “/usr/bin/example/”,50); printf(“Directory name before modifying = ” \ “%s\n”, getenv(“DIR”)); putenv(“DIR=/usr/home/”); printf(“Directory name after modifying = ” \ “%s\n”, getenv(“DIR”)); return 0; }

Output:

Directory name before modifying = /usr/bin/example/Directory name after modifying = /usr/home/

Difference between variable declaration & definition in C:

S.no Variable declaration Variable definition 1 Declaration tells the compiler about data type and size of

the variable. Definition allocates memory for the variable.

2 Variable can be declared many times in a program. It can happen only one time for a variable in a program.

3 The assignment of properties and identification to a variable.

Assignments of storage space to a variable.

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C – Operators and Expressions

The symbols which are used to perform logical and mathematical operations in a C program are called C operators.

• These C operators join individual constants and variables to form expressions. • Operators, functions, constants and variables are combined together to form expressions. • Consider the expression A + B * 5. where, +, * are operators, A, B are variables, 5 is constant and A + B * 5

is an expression.

Types of C operators: C language offers many types of operators. They are,

1. Arithmetic operators 2. Assignment operators 3. Relational operators 4. Logical operators 5. Bit wise operators 6. Conditional operators (ternary operators) 7. Increment/decrement operators 8. Special operators

Continue on types of C operators:

S.no Types of Operators Description

1 Arithmetic_operators

These are used to perform mathematical calculations like addition, subtraction, multiplication, division and modulus

2 Assignment_operators These are used to assign the values for the variables in C programs.

3 Relational operators These operators are used to compare the value of two variables.

4 Logical operators These operators are used to perform logical operations on the given two variables.

5 Bit wise operators These operators are used to perform bit operations on given two variables.

6 Conditional (ternary) operators

Conditional operators return one value if condition is true and returns another value is condition is false.

7 Increment/decrement operators

These operators are used to either increase or decrease the value of the variable by one.

8 Special operators &, *, sizeof( ) and ternary operators.

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C – Decision Control statement

In decision control statements (C if else and nested if), group of statements are executed when condition is true. If condition is false, then else part statements are executed.

• There are 3 types of decision making control statements in C language. They are,

1. if statements 2. if else statements 3. nested if statements

“If”, “else” and “nested if” decision control statements in C:

• Syntax for each C decision control statements are given in below table with description.

Decision control

statements

Syntax Description

if if (condition) { Statements; }

In these type of statements, if condition is true, then respective block of code is executed.

if…else if (condition) { Statement1; Statement2; } else { Statement3; Statement4; }

In these type of statements, group of statements are executed when condition is true. If condition is false, then else part statements are executed.

nested if if (condition1){ Statement1; } else_if(condition2) { Statement2; } else Statement 3;

If condition 1 is false, then condition 2 is checked and statements are executed if it is true. If condition 2 also gets failure, then else part is executed.

Example program for if statement in C:

In “if” control statement, respective block of code is executed when condition is true.

int main() { int m=40,n=40; if (m == n) { printf("m and n are equal"); } }

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Output:

m and n are equal

Example program for if else statement in C:

In C if else control statement, group of statements are executed when condition is true. If condition is false, then else part statements are executed.

#include <stdio.h> int main() { int m=40,n=20; if (m == n) { printf("m and n are equal"); } else { printf("m and n are not equal"); } }

Output:

m and n are not equal

Example program for nested if statement in C:

• In “nested if” control statement, if condition 1 is false, then condition 2 is checked and statements are executed if it is true.

• If condition 2 also gets failure, then else part is executed.

#include <stdio.h> int main() { int m=40,n=20; if (m>n) { printf("m is greater than n"); } else if(m<n) { printf("m is less than n"); } else { printf("m is equal to n"); } }

Output:

m is greater than n

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C – Loop control statements

Loop control statements in C are used to perform looping operations until the given condition is true. Control comes out of the loop statements once condition becomes false.

Types of loop control statements in C:

There are 3 types of loop control statements in C language. They are,

1. for 2. while 3. do-while

• Syntax for each C loop control statements are given in below table with description.

S.no Loop Name

Syntax Description

1 for for (exp1; exp2; expr3) { statements; }

Where, exp1 – variable initialization ( Example: i=0, j=2, k=3 )exp2 – condition checking ( Example: i>5, j<3, k=3 )exp3 – increment/decrement ( Example: ++i, j–, ++k )

2 while while (condition) { statements; }

where, condition might be a>5, i<10

3 do while do { statements; } while (condition);

where, condition might be a>5, i<10

Example program (for loop) in C:

In for loop control statement, loop is executed until condition becomes false.

#include <stdio.h> int main() { int i; for(i=0;i<10;i++) { printf("%d ",i); } }

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Output:

0 1 2 3 4 5 6 7 8 9

Example program (while loop) in C:

In while loop control statement, loop is executed until condition becomes false.

#include <stdio.h> int main() { int i=3; while(i<10) { printf("%d\n",i); i++; } }

Output:

3 4 5 6 7 8 9

Example program (do while loop) in C:

In do..while loop control statement, while loop is executed irrespective of the condition for first time. Then 2nd time onwards, loop is executed until condition becomes false.

#include <stdio.h> int main() { int i=1; do { printf("Value of i is %d\n",i); i++; }while(i<=4 && i>=2); }

Output:

Value of i is 1 Value of i is 2 Value of i is 3 Value of i is 4

Difference between while & do while loops in C:

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S.no while do while 1 Loop is

executed only when condition is true.

Loop is executed for first time irrespective of the condition. After executing while loop for first time, then condition is checked.

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C – Case control statements

The statements which are used to execute only specific block of statements in a series of blocks are called case control statements.

There are 4 types of case control statements in C language. They are,

1. switch 2. break 3. continue 4. goto

1. switch case statement in C:

• Switch case statements are used to execute only specific case statements based on the switch expression. • Below is the syntax for switch case statement.

switch (expression) { case label1: statements; break; case label2: statements; break; default: statements; break; }

Example program for switch..case statement in C:

#include <stdio.h> int main () { int value = 3; switch(value) { case 1: printf(“Value is 1 \n” ); break; case 2: printf(“Value is 2 \n” ); break; case 3: printf(“Value is 3 \n” ); break; case 4: printf(“Value is 4 \n” ); break; default : printf(“Value is other than 1,2,3,4 \n” ); } return 0; }

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Output:

Value is 3

2. break statement in C:

• Break statement is used to terminate the while loops, switch case loops and for loops from the subsequent execution.

• Syntax: break;

Example program for break statement in C:

#include <stdio.h> int main() { int i;

for(i=0;i<10;i++) { if(i==5) { printf(“\nComing out of for loop when i = 5″); break; } printf(“%d “,i); } }

Output:

0 1 2 3 4 Coming out of for loop when i = 5

3. Continue statement in C:

• Continue statement is used to continue the next iteration of for loop, while loop and do-while loops. So, the remaining statements are skipped within the loop for that particular iteration.

• Syntax : continue;

Example program for continue statement in C:

#include <stdio.h> int main() { int i; for(i=0;i<10;i++) { if(i==5 || i==6) { printf(“\nSkipping %d from display using ” \ “continue statement \n”,i); continue; } printf(“%d “,i);

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} }

Output:

0 1 2 3 4 Skipping 5 from display using continue statementSkipping 6 from display using continue statement7 8 9

4. goto statement in C:

• goto statements is used to transfer the normal flow of a program to the specified label in the program. • Below is the syntax for goto statement in C.

{ ……. go to label; ……. ……. LABEL: statements; }

Example program for goto statement in C:

#include <stdio.h> int main() { int i; for(i=0;i<10;i++) { if(i==5) { printf(“\nWe are using goto statement when i = 5″); goto HAI; } printf(“%d “,i); } HAI : printf(“\nNow, we are inside label name \”hai\” \n”); }

Output:

0 1 2 3 4 We are using goto statement when i = 5 Now, we are inside label name “hai”

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C – Type Qualifiers

C – type qualifiers : The keywords which are used to modify the properties of a variable are called type qualifiers.

Types of C type qualifiers:

There are two types of qualifiers available in C language. They are,

1. const 2. volatile

1. const keyword:

• Constants are also like normal variables. But, only difference is, their values can’t be modified by the program once they are defined.

• They refer to fixed values. They are also called as literals. • They may be belonging to any of the data type.

• Syntax:

const data_type variable_name; (or) const data_type *variable_name;

• Please refer C – Constants topic in this tutorial for more details on const keyword.

2. volatile keyword:

• When a variable is defined as volatile, the program may not change the value of the variable explicitly. • But, these variable values might keep on changing without any explicit assignment by the program. These

types of qualifiers are called volatile. • For example, if global variable’s address is passed to clock routine of the operating system to store the

system time, the value in this address keep on changing without any assignment by the program. These variables are named as volatile variable.

• Syntax:

volatile data_type variable_name; (or) volatile data_type *variable_name;

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C – Storage Class Specifiers

Storage class specifiers in C language tells the compiler where to store a variable, how to store the variable, what is the initial value of the variable and life time of the variable.

Syntax: storage_specifier data_type variable _name

Types of Storage Class Specifiers in C:

There are 4 storage class specifiers available in C language. They are,

1. auto 2. extern 3. static 4. register

S. No.

Storage Specifier

Storage place

Initial / default value

Scope Life

1 auto CPU Memory

Garbage value local Within the function only.

2 extern CPU memory

Zero Global Till the end of the main program. Variable definition might be anywhere in the C program

3 static CPU memory

Zero local Retains the value of the variable between different function calls.

4 register Register memory

Garbage value local Within the function

Note:

• For faster access of a variable, it is better to go for register specifiers rather than auto specifiers. • Because, register variables are stored in register memory whereas auto variables are stored in main CPU

memory. • Only few variables can be stored in register memory. So, we can use variables as register that are used very

often in a C program.

Example program for auto variable in C:

The scope of this auto variable is within the function only. It is equivalent to local variable. All local variables are auto variables by default.

#include<stdio.h> void increment(void); int main() { increment(); increment(); increment();

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increment(); return 0; } void increment(void) { auto int i = 0 ; printf ( “%d “, i ) ; i++; }

Output:

0 0 0 0

Example program for static variable in C:

Static variables retain the value of the variable between different function calls.

//C static example #include<stdio.h> void increment(void); int main() { increment(); increment(); increment(); increment(); return 0; } void increment(void) { static int i = 0 ; printf ( “%d “, i ) ; i++; }

Output:

0 1 2 3

Example program for extern variable in C:

The scope of this extern variable is throughout the main program. It is equivalent to global variable. Definition for extern variable might be anywhere in the C program.

#include<stdio.h> int x = 10 ; int main( ) { extern int y; printf(“The value of x is %d \n”,x); printf(“The value of y is %d”,y); return 0; } int y=50;

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Output:

The value of x is 10 The value of y is 50

Example program for register variable in C:

• Register variables are also local variables, but stored in register memory. Whereas, auto variables are stored in main CPU memory.

• Register variables will be accessed very faster than the normal variables since they are stored in register memory rather than main memory.

• But, only limited variables can be used as register since register size is very low. (16 bits, 32 bits or 64 bits)

#include <stdio.h> int main() { register int i; int arr[5];// declaring array arr[0] = 10;// Initializing array arr[1] = 20; arr[2] = 30; arr[3] = 40; arr[4] = 50; for (i=0;i<5;i++) { // Accessing each variable printf(“value of arr[%d] is %d \n”, i, arr[i]); } return 0; }

Output:

value of arr[0] is 10 value of arr[1] is 20 value of arr[2] is 30 value of arr[3] is 40 value of arr[4] is 50

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C – Array

C Array is a collection of variables belongings to the same data type. You can store group of data of same data type in an array.

• Array might be belonging to any of the data types • Array size must be a constant value. • Always, Contiguous (adjacent) memory locations are used to store array elements in memory. • It is a best practice to initialize an array to zero or null while declaring, if we don’t assign any values to array.

Example for C Arrays:

• int a[10]; // integer array • char b[10]; // character array i.e. string

Types of C arrays:

There are 2 types of C arrays. They are,

1. One dimensional array 2. Multi dimensional array

1. Two dimensional array 2. Three dimensional array, four dimensional array etc…

1. One dimensional array in C:

• Syntax : data-type arr_name[array_size];

Array declaration

Array initialization Accessing array

Syntax: data_type arr_name [arr_size];

data_type arr_name [arr_size]=(value1, value2, value3,….);

arr_name[index];

int age [5]; int age[5]={0, 1, 2, 3, 4}; age[0];_/*0_is_accessed*/age[1];_/*1_is_accessed*/age[2];_/*2_is_accessed*/

char str[10]; char str[10]={‘H’,‘a’,‘i’}; (or)char str[0] = ‘H’;char str[1] = ‘a’;

char str[2] = ‘i;

str[0];_/*H is accessed*/str[1]; /*a is accessed*/str[2]; /* i is accessed*/

Example program for one dimensional array in C:

#include<stdio.h>int main(){

int i;

int arr[5] = {10,20,30,40,50};

// declaring and Initializing array in C

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//To initialize all array elements to 0, use int arr[5]={0};

/* Above array can be initialized as below also

arr[0] = 10;

arr[1] = 20;

arr[2] = 30;

arr[3] = 40;

arr[4] = 50; */

for (i=0;i<5;i++)

{

// Accessing each variable

printf(“value of arr[%d] is %d \n”, i, arr[i]);

}

}

Output:


2. Two dimensional array in C:

• Two dimensional array is nothing but array of array. • syntax : data_type array_name[num_of_rows][num_of_column]

S.no Array declaration Array initialization Accessing array 1 Syntax: data_type arr_name

[num_of_rows][num_of_column]; data_type arr_name[2][2] = {{0,0},{0,1},{1,0},{1,1}};

arr_name[index];

2 Example:int arr[2][2]; int arr[2][2] = {1,2, 3, 4}; arr [0] [0] = 1; arr [0] ]1] = 2;arr [1][0] = 3;

arr [1] [1] = 4;

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Example program for two dimensional array in C:

#include<stdio.h>int main(){

int i,j;

// declaring and Initializing array

int arr[2][2] = {10,20,30,40};

/* Above array can be initialized as below also

arr[0][0] = 10; // Initializing array

arr[0][1] = 20;

arr[1][0] = 30;

arr[1][1] = 40; */

for (i=0;i<2;i++)

{

for (j=0;j<2;j++)

{

// Accessing variables

printf(“value of arr[%d] [%d] : %d\n”,i,j,arr[i][j]);

}

}

}

Output:

value of arr[0] [0] is 10 value of arr[0] [1] is 20 value of arr[1] [0] is 30 value of arr[1] [1] is 40

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C – String

• C Strings are nothing but array of characters ended with null character (‘\0’). • This null character indicates the end of the string. • Strings are always enclosed by double quotes. Whereas, character is enclosed by single quotes in C.

Example for C string:

• char string[20] = { ‘f’ , ’r’ , ‘e’ , ‘s’ , ‘h’ , ‘2’ , ‘r’ , ‘e’ , ‘f’ , ’r’ , ‘e’ , ‘s’ , ‘h’ , ‘\0’}; (or) • char string[20] = “fresh2refresh”; (or) • char string [] = “fresh2refresh”;

• Difference between above declarations are, when we declare char as “string[20]“, 20 bytes of memory space is allocated for holding the string value.

• When we declare char as “string[]”, memory space will be allocated as per the requirement during execution of the program.

Example program for C string:

#include <stdio.h> int main () { char string[20] = "fresh2refresh.com"; printf("The string is : %s \n", string ); return 0; }

Output:

The string is : fresh2refresh.com

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C String functions:

• String.h header file supports all the string functions in C language. All the string functions are given below. • Click on each string function name below for detail description and example programs.

S.no String functions

Description

1 strcat ( ) Concatenates str2 at the end of str1. 2 strncat ( ) appends a portion of string to another 3 strcpy ( ) Copies str2 into str1 4 strncpy ( ) copies given number of characters of one string to another 5 strlen ( ) gives the length of str1. 6 strcmp ( ) Returns 0 if str1 is same as str2. Returns <0 if strl < str2. Returns >0 if str1 > str2. 7 strcmpi_(.) Same as strcmp() function. But, this function negotiates case. “A” and “a” are treated as

same. 8 strchr ( ) Returns pointer to first occurrence of char in str1. 9 strrchr ( ) last occurrence of given character in a string is found

10 strstr ( ) Returns pointer to first occurrence of str2 in str1. 11 strrstr ( ) Returns pointer to last occurrence of str2 in str1. 12 strdup ( ) duplicates the string 13 strlwr ( ) converts string to lowercase 14 strupr ( ) converts string to uppercase 15 strrev ( ) reverses the given string 16 strset ( ) sets all character in a string to given character 17 strnset ( ) It sets the portion of characters in a string to given character 18 strtok ( ) tokenizing given string using delimiter

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C – Pointer

• C Pointer is a variable that stores/points the address of another variable. C Pointer is used to allocate memory dynamically i.e. at run time. The pointer variable might be belonging to any of the data type such as int, float, char, double, short etc.

Syntax : data_type *var_name; Example : int *p; char *p;

• Where, * is used to denote that “p” is pointer variable and not a normal variable.

Key points to remember about pointers in C:

• Normal variable stores the value whereas pointer variable stores the address of the variable. • The content of the C pointer always be a whole number i.e. address. • Always C pointer is initialized to null, i.e. int *p = null. • The value of null pointer is 0.

• & symbol is used to get the address of the variable. • * symbol is used to get the value of the variable that the pointer is pointing to. • If pointer is assigned to NULL, it means it is pointing to nothing. • Two pointers can be subtracted to know how many elements are available between these two pointers.

• But, Pointer addition, multiplication, division are not allowed. • The size of any pointer is 2 byte (for 16 bit compiler).

Example program for pointer in C:

#include <stdio.h> int main() { int *ptr, q; q = 50; /* address of q is assigned to ptr */ ptr = &q; /* display q’s value using ptr variable */ printf(“%d”, *ptr); return 0; } . Output:

50 .

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C – Function

C functions are basic building blocks in a program. All C programs are written using functions to improve re-usability, understandability and to keep track on them. You can learn below concepts of C functions in this section in detail.

1. What is C function? 2. Uses of C functions 3. C function declaration, function call and definition with example program 4. How to call C functions in a program?

1. Call by value 2. Call by reference

5. C function arguments and return values 1. C function with arguments and with return value 2. C function with arguments and without return value 3. C function without arguments and without return value 4. C function without arguments and with return value

6. Types of C functions 1. Library functions in C 2. User defined functions in C

1. Creating/Adding user defined function in C library 7. Command line arguments in C 8. Variable length arguments in C

1. What is C function?

A large C program is divided into basic building blocks called C function. C function contains set of instructions enclosed by “{ }” which performs specific operation in a C program. Actually, Collection of these functions creates a C program.

2. Uses of C functions:

• C functions are used to avoid rewriting same logic/code again and again in a program. • There is no limit in calling C functions to make use of same functionality wherever required. • We can call functions any number of times in a program and from any place in a program. • A large C program can easily be tracked when it is divided into functions. • The core concept of C functions are, re-usability, dividing a big task into small pieces to achieve the

functionality and to improve understandability of very large C programs.

3. C function declaration, function call and function definition:

There are 3 aspects in each C function. They are,

• Function declaration or prototype - This informs compiler about the function name, function parameters and return value’s data type.

• Function call – This calls the actual function • Function definition – This contains all the statements to be executed.

S.no C function aspects syntax 1 function definition return_type function_name ( arguments list )

{ Body of function; }

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2 function call function_name ( arguments list ); 3 function declaration return_type function_name ( argument list );

Simple example program for C function:

• As you know, functions should be declared and defined before calling in a C program. • In the below program, function “square” is called from main function. • The value of “m” is passed as argument to the function “square”. This value is multiplied by itself in this

function and multiplied value “p” is returned to main function from function “square”.

#include<stdio.h> // function prototype, also called function declaration float square ( float x ); // main function, program starts from here int main( ) { float m, n ; printf ( "\nEnter some number for finding square \n"); scanf ( "%f", &m ) ; // function call n = square ( m ) ; printf ( "\nSquare of the given number %f is %f",m,n ); } float square ( float x ) // function definition { float p ; p = x * x ; return ( p ) ; }

Output:

Enter some number for finding square 2 Square of the given number 2.000000 is 4.000000

4. How to call C functions in a program?

There are two ways that a C function can be called from a program. They are,

1. Call by value 2. Call by reference

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1. Call by value:

• In call by value method, the value of the variable is passed to the function as parameter. • The value of the actual parameter can not be modified by formal parameter. • Different Memory is allocated for both actual and formal parameters. Because, value of actual

parameter is copied to formal parameter.

Note:

• Actual parameter – This is the argument which is used in function call. • Formal parameter – This is the argument which is used in function definition

Example program for C function (using call by value):

• In this program, the values of the variables “m” and “n” are passed to the function “swap”. • These values are copied to formal parameters “a” and “b” in swap function and used.

#include<stdio.h> // function prototype, also called function declaration void swap(int a, int b); int main() { int m = 22, n = 44; // calling swap function by value printf(" values before swap m = %d \nand n = %d", m, n); swap(m, n); } void swap(int a, int b) { int tmp; tmp = a; a = b; b = tmp; printf(" \nvalues after swap m = %d\n and n = %d", a, b); }

Output:

values before swap m = 22 and n = 44 values after swap m = 44 and n = 22

2. Call by reference:

• In call by reference method, the address of the variable is passed to the function as parameter. • The value of the actual parameter can be modified by formal parameter. • Same memory is used for both actual and formal parameters since only address is used by both

parameters.

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Example program for C function (using call by reference):

• In this program, the address of the variables “m” and “n” are passed to the function “swap”. • These values are not copied to formal parameters “a” and “b” in swap function. • Because, they are just holding the address of those variables. • This address is used to access and change the values of the variables.

#include<stdio.h> // function prototype, also called function declaration void swap(int *a, int *b); int main() { int m = 22, n = 44; // calling swap function by reference printf("values before swap m = %d \n and n = %d",m,n); swap(&m, &n); } void swap(int *a, int *b) { int tmp; tmp = *a; *a = *b; *b = tmp; printf("\n values after swap a = %d \nand b = %d", *a, *b); }

Output:

values before swap m = 22 and n = 44 values after swap a = 44 and b = 22

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C – Argument, return value

All C functions can be called either with arguments or without arguments in a C program. These functions may or may not return values to the calling function. Now, we will see simple example C programs for each one of the below.

1. C function with arguments (parameters) and with return value 2. C function with arguments (parameters) and without return value 3. C function without arguments (parameters) and without return value 4. C function without arguments (parameters) and with return value

S.no C function syntax1 with arguments and with

return values int function ( int ); // function declaration function ( a ); // function call int function( int a ) // function definition {statements; return a;}

2 with arguments and without return values

void function ( int ); // function declaration function( a ); // function call void function( int a ) // function definition {statements;}

3 without arguments and without return values

void function(); // function declaration function(); // function call void function() // function definition {statements;}

4 without arguments and with return values

int function ( ); // function declaration function ( ); // function call int function( ) // function definition {statements; return a;}

Note:

• If the return data type of a function is “void”, then, it can’t return any values to the calling function. • If the return data type of the function is other than void such as “int, float, double etc”, then, it can return

values to the calling function.

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1. Example program for with arguments & with return value:

In this program, integer, array and string are passed as arguments to the function. The return type of this function is “int” and value of the variable “a” is returned from the function. The values for array and string are modified inside the function itself.

#include<stdio.h> #include<string.h> int function(int, int[], char[]); int main() { int i, a = 20; int arr[5] = {10,20,30,40,50}; char str[30] = "\"fresh2refresh\""; printf(" ***values before modification***\n"); printf("value of a is %d\n",a); for (i=0;i<5;i++) { // Accessing each variable printf("value of arr[%d] is %d\n",i,arr[i]); } printf("value of str is %s\n",str); printf("\n ***values after modification***\n"); a= function(a, &arr[0], &str[0]); printf("value of a is %d\n",a); for (i=0;i<5;i++) { // Accessing each variable printf("value of arr[%d] is %d\n",i,arr[i]); } printf("value of str is %s\n",str); return 0; } int function(int a, int *arr, char *str) { int i; a = a+20; arr[0] = arr[0]+50; arr[1] = arr[1]+50; arr[2] = arr[2]+50; arr[3] = arr[3]+50; arr[4] = arr[4]+50; strcpy(str,"\"modified string\""); return a; }

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Output:

***values before modification*** value of a is 20 value of arr[0] is 10 value of arr[1] is 20 value of arr[2] is 30 value of arr[3] is 40 value of arr[4] is 50 value of str is “fresh2refresh”***values after modification***value of a is 40 value of arr[0] is 60 value of arr[1] is 70 value of arr[2] is 80 value of arr[3] is 90 value of arr[4] is 100 value of str is “modified string”

2. Example program for with arguments & without return value:

In this program, integer, array and string are passed as arguments to the function. The return type of this function is “void” and no values can be returned from the function. All the values of integer, array and string are manipulated and displayed inside the function itself.

#include<stdio.h> void function(int, int[], char[]); int main() { int a = 20; int arr[5] = {10,20,30,40,50}; char str[30] = "\"fresh2refresh\""; function(a, &arr[0], &str[0]); return 0; } void function(int a, int *arr, char *str) { int i; printf("value of a is %d\n\n",a); for (i=0;i<5;i++) { // Accessing each variable printf("value of arr[%d] is %d\n",i,arr[i]); } printf("\nvalue of str is %s\n",str); }

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Output:

value of a is 20


value of str is “fresh2refresh”

3. Example program for without arguments & without return value:

In this program, no values are passed to the function “test” and no values are returned from this function to main function.

#include<stdio.h> void test(); int main() { test(); return 0; } void test() { int a = 50, b = 80; printf("\nvalues : a = %d and b = %d", a, b); }

Output:

values : a = 50 and b = 80

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4. Example program for without arguments & with return value:

In this program, no arguments are passed to the function “sum”. But, values are returned from this function to main function. Values of the variable a and b are summed up in the function “sum” and the sum of these value is returned to the main function.

#include<stdio.h> int sum(); int main() { int addition; addition = sum(); printf("\nSum of two given values = %d", addition); return 0; } int sum() { int a = 50, b = 80, sum; sum = a + b; return sum; }

Output:

Sum of two given values = 130

Do you know how many values can be return from C functions?

• Always, Only one value can be returned from a function. • If you try to return more than one values from a function, only one value will be returned that appears at

the right most place of the return statement. • For example, if you use “return a,b,c” in your function, value for c only will be returned and values a, b

won’t be returned to the program. • In case, if you want to return more than one values, pointers can be used to directly change the values

in address instead of returning those values to the function.

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C – Library functions

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• Library functions in C language are inbuilt functions which are grouped together and placed in a common place called library.

• Each library function in C performs specific operation. • We can make use of these library functions to get the pre-defined output instead of writing our own

code to get those outputs. • These library functions are created by the persons who designed and created C compilers. • All C standard library functions are declared in many header files which are saved as file_name.h. • Actually, function declaration, definition for macros are given in all header files. • We are including these header files in our C program using “#include<file_name.h>” command to make

use of the functions those are declared in the header files. • When we include header files in our C program using “#include<filename.h>” command, all C code of

the header files are included in C program. Then, this C program is compiled by compiler and executed.

1. C – stdio.h source code

2. C – conio.h source code

3. C – string.h source code

4. C – stdlib.h source code

5. C – math.h source code

6. C – time.h source code

7. C – ctype.h source code

• If you want to check source code for all header files, you can check inside “include” directory after C compiler is installed in your machine.

• For example, if you install DevC++ compiler in C directory in your machine, “C:\Dev-Cpp\include” is the path where all header files will be available.

List of most used header files in C:

• Check the below table to know all the C library functions and header files in which they are declared.

Click on the each header file name below to know the list of inbuilt functions declared inside them.

S.No Header file Description 1 stdio.h This is standard input/output header file in which Input/Output

functions are declared 2 conio.h This is console input/output header file 3 string.h All string related functions are defined in this header file 4 stdlib.h This header file contains general functions used in C programs5 math.h All maths related functions are defined in this header file

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6 time.h This header file contains time and clock related functions 7 ctype.h All character handling functions are defined in this header file 8 stdarg.h Variable argument functions are declared in this header file 9 signal.h Signal handling functions are declared in this file

10 setjmp.h This file contains all jump functions 11 locale.h This file contains locale functions 12 errno.h Error handling functions are given in this file 13 assert.h This contains diagnostics functions

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C – Creating library functions

User defined functions in C:

• As you know, there are 2 types of functions in C. They are, library functions and user defined functions. • Library functions are inbuilt functions which are available in common place called C library. Where as, User

defined functions are the functions which are written by us for our own requirement.

Adding user defined functions in C library:

• Do you know that we can add our own user defined functions in C library? • Yes. It is possible to add, delete, modify and access our own user defined function to or from C library.

• The advantage of adding user defined function in C library is, this function will be available for all C programs once added to the C library.

• We can use this function in any C program as we use other C library functions. • In latest version of GCC compilers, compilation time can be saved since these functions are available in

library in the compiled form. • Normal header files are saved as ”file_name.h” in which all library functions are available. These header files

contain source code and this source code is added in main C program file where we add this header file using “#include <file_name.h>” command.

• Where as, precompiled version of header files are saved as “file_name.gch”.

Steps for adding our own functions in C library:

Step 1:

For example, below is a sample function that is going to be added in the C library. Write the below function in a file and save it as “addition.c”

addition(int i, int j) { int total; total = i + j; return total; }

Step 2:

Compile “addition.c” file by using Alt + F9 keys (in turbo C).

step 3:

“addition.obj” file would be created which is the compiled form of “addition.c” file.

Step 4:

Use the below command to add this function to library (in turbo C). c:\> tlib math.lib + c:\ addition.obj + means adding c:\addition.obj file in the math library. We can delete this file using – (minus).

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Step 5:

Create a file “addition.h” & declare prototype of addition() function like below. int addition (int i, int j); Now addition.h file containing prototype of function “addition”.

Note : Please create, compile and add files in the respective directory as directory name may change for each IDE.

Step 6:

Let us see how to use our newly added library function in a C program.

# include <stdio.h> // Including our user defined function. # include “c:\\addition.h” int main () { int total; // calling function from library total = addition (10, 20); printf ("Total = %d \n", total); }

Output:

Total = 30

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C – Command line arguments

Command line arguments in C:

main() function of a C program accepts arguments from command line or from other shell scripts by following commands. They are,

• argc • argv[]

where,

argc - Number of arguments in the command line including program name argv[] – This is carrying all the arguments

• In real time application, it will happen to pass arguments to the main program itself. These arguments are passed to the main () function while executing binary file from command line.

• For example, when we compile a program (test.c), we get executable file in the name “test”. • Now, we run the executable “test” along with 4 arguments in command line like below.

./test this is a program

Where,

argc = 5 argv[0] = “test” argv[1] = “this” argv[2] = “is” argv[3] = “a” argv[4] = “program” argv[5] = NULL

Example program for argc() and argv() functions in C:

#include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) // command line arguments { if(argc!=5) { printf("Arguments passed through command line " \ "not equal to 5"); return 1; } printf("\n Program name : %s \n", argv[0]); printf("1st arg : %s \n", argv[1]); printf("2nd arg : %s \n", argv[2]); printf("3rd arg : %s \n", argv[3]); printf("4th arg : %s \n", argv[4]); printf("5th arg : %s \n", argv[5]);

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return 0; }

Output:

Program name : test 1st arg : this 2nd arg : is 3rd arg : a 4th arg : program 5th arg : (null)

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C – Variable length argument

• Variable length arguments is an advanced concept in C language offered by c99 standard. In c89 standard, fixed arguments only can be passed to the functions.

• When a function gets number of arguments that changes at run time, we can go for variable length arguments.

• It is denoted as … (3 dots) • stdarg.h header file should be included to make use of variable length argument functions.

Example program for variable length arguments in C:

#include <stdio.h> #include <stdarg.h> int add(int num,...); int main() { printf("The value from first function call = " \ "%d\n", add(2,2,3)); printf("The value from second function call= " \ "%d \n", add(4,2,3,4,5)); /*Note - In function add(2,2,3), first 2 is total number of arguments 2,3 are variable length arguments In function add(4,2,3,4,5), 4 is total number of arguments 2,3,4,5 are variable length arguments */ return 0; } int add(int num,...) { va_list valist; int sum = 0; int i; va_start(valist, num); for (i = 0; i < num; i++) { sum += va_arg(valist, int); } va_end(valist); return sum; }

Output:

The value from first function call = 5 The value from second function call= 14

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In the above program, function “add” is called twice. But, number of arguments passed to the function gets varies for each. So, 3 dots (…) are mentioned for function ‘add” that indicates that this function will get any number of arguments at run time.

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C – Summary of C functions

As you know, C functions are basic building blocks in every C program. We have given key points those to be kept in mind for using existing C library functions and writing our own functions in a C program efficiently.

Key points to remember while writing functions in C:

• All C programs contain main() function which is mandatory. • main() function is the function from where every C program is started to execute. • Name of the function is unique in a C program. • C Functions can be invoked from anywhere within a C program. • There can any number of functions be created in a program. There is no limit on this.

• There is no limit in calling C functions in a program. • All functions are called in sequence manner specified in main() function. • One function can be called within another function. • C functions can be called with or without arguments/parameters. These arguments are nothing but inputs to

the functions. • C functions may or may not return values to calling functions. These values are nothing but output of the

functions.

• When a function completes its task, program control is returned to the function from where it is called. • There can be functions within functions. • Before calling and defining a function, we have to declare function prototype in order to inform the compiler

about the function name, function parameters and return value type.

• C function can return only one value to the calling function. • When return data type of a function is “void”, then, it won’t return any values • When return data type of a function is other than void such as “int, float, double”, it returns value to the

calling function. • main() program comes to an end when there is no functions or commands to execute. • There are 2 types of functions in C. They are, 1. Library functions 2. User defined functions

There are many inbuilt C functions which are offered by C compiler. You can check list of all C functions and simple example programs with outputs below.

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C – Arithmetic functions

• C functions which are used to perform mathematical operations in a program are called Arithmetic functions. • Example program for abs(), floor(), round(), ceil(), sqrt(), exp(), log(), sin(), cos(), tan(), pow() and trunc()

functions are given below. • If you want to know what is the structure and declaration of a C function, Please refer “C_Function“ topic in

this tutorial.

List of inbuilt arithmetic functions in C language:

• “math.h” and “stdlib.h” header files support all the arithmetic functions in C language. All the arithmetic functions used in C language are given below.

• Click on each function name below for detail description and example programs.

S.no Function Description 1 abs ( ) This function returns the absolute value of an integer. The absolute value of a number is always

positive. Only integer values are supported in C. 2 floor ( ) This function returns the nearest integer which is less than or equal to the argument passed to this

function. 3 round.(.) This function returns the nearest integer value of the float/double/long double argument passed to

this function. If decimal value is from ”.1 to .5″, it returns integer value less than the argument. If decimal value is from “.6 to .9″, it returns the integer value greater than the argument.

4 ceil ( ) This function returns nearest integer value which is greater than or equal to the argument passed to this function.

5 sin ( ) This function is used to calculate sine value. 6 cos ( ) This function is used to calculate cosine. 7 cosh ( ) This function is used to calculate hyperbolic cosine. 8 exp ( ) This function is used to calculate the exponential “e” to the xth power. 9 tan ( ) This function is used to calculate tangent.

10 tanh ( ) This function is used to calculate hyperbolic tangent. 11 sinh ( ) This function is used to calculate hyperbolic sine. 12 log ( ) This function is used to calculates natural logarithm. 13 log10 ( ) This function is used to calculates base 10 logarithm. 14 sqrt ( ) This function is used to find square root of the argument passed to this function. 15 pow ( ) This is used to find the power of the given number. 16 trunc.(.) This function truncates the decimal value from floating point value and returns integer value.

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C – Int, char validation functions

There are many inbuilt functions in C language which are used to validate the data type of given variable and to convert upper to lower case and lower to upper case are given below with description and simple example programs.

List of inbuilt int, char validation functions in C language:

• “ctype.h” header file support all the below functions in C language.

S.no Function Description

1 isalpha() checks whether character is alphabetic

2 isdigit() checks whether character is digit

3 isalnum() checks whether character is alphanumeric

4 isspace() checks whether character is space

5 islower() checks whether character is lower case

6 isupper() checks whether character is upper case

7 isxdigit() checks whether character is hexadecimal

8 iscntrl() checks whether character is a control character

9 isprint() checks whether character is a printable character

10 ispunct() checks whether character is a punctuation

11 isgraph() checks whether character is a graphical character

12 tolower() checks whether character is alphabetic & converts to lower case

13 toupper() checks whether character is alphabetic & converts to upper case

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C – Buffer manipulation function

• Buffer manipulation functions in C work on the address of the memory block rather than the values inside the address.

• Example programs for memset(), memcpy(), memmove(), memcmp(), memicmp() and memchr() functions are given below.

S.no Function Description 1 memset() It is used to initialize a specified number of bytes to null or any other value in the buffer 2 memcpy() It is used to copy a specified number of bytes from one memory to another 3 memmove() It is used to copy a specified number of bytes from one memory to another or to overlap on same

memory. Difference between memmove and memcpy is, overlap can happen on memmove whereas memcpy should be done in non-destructive way

4 memcmp() It is used to compare specified number of characters from two buffers 5 memicmp() It is used to compare specified number of characters from two buffers regardless of the case of

the characters 6 memchr() It is used to locate the first occurrence of the character in the specified string

Example program for memset() function in C:

• memset( ) function is used to initialize specified number of bytes to null or to any other value in the buffer.

#include <stdio.h> #include <string.h> #include <stdlib.h> int main() { int i; /* allocate memory for array of 5 elements */ char *a = (char *) malloc(5*sizeof(char)); printf("Values before memset\n"); for (i = 0; i < 5; ++i) printf(" a[%d] = %d ,", i, a[i]); /* All elements are set to 3. It can be set to any value */ memset(a, 3, 5*sizeof(char)); printf("\nValues after memset\n"); for (i = 0; i < 5; ++i) printf(" a[%d] = %d ,", i, a[i]); // remove x from memory free(a); return 0; }

Output:

Values before memset a[0] = 0 , a[1] = 0 , a[2] = 0 , a[3] = 0 , a[4] = 0Values after memset

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a[0] = 3 , a[1] = 3 , a[2] = 3 , a[3] = 3 , a[4] = 3

Example program for memcpy() function in C:

• memcpy( ) function is used to copy a specified number of bytes from one memory to another.

#include <stdio.h> #include <string.h> int main() { // define two identical arrays char str1[10] = "fresh"; char str2[10] ; if (memcpy(str2,str1, strlen(str1))) { printf("Elements in str1 are copied to str2 .\n"); printf("str1 = %s\n str2 = %s \n", str1, str2); } else printf("Error while coping str1 into str2.\n"); return 0; }

Output:

Elements in str1 are copied to str2 . str1 = fresh str2 = fresh

Example program for memmove() function in C:

• memmove( ) function is used to copy a specified number of bytes from one memory to another or to overlap on same memory.

#include <stdio.h> #include <string.h> int main() { // define two identical arrays char str1[10] = "fresh"; printf("str1 before memmove\n"); printf("str1 = %s\n ", str1); if (memmove(str1+2,str1, strlen(str1))) { printf("Elements in str1 are moved/overlapped on str1.\n"); printf("str1 = %s \n", str1); } else printf("Error while coping str1 into str2.\n"); return 0; }

Output:

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str1 before memmove str1 = fresh Elements in str1 are moved/overlapped on str1 .str1 = frfresh

Example program for memcmp() function in C:

• memcmp( ) function is used to compare specified number of characters from two buffers.

#include <stdio.h> #include <string.h> int main() { // define two identical arrays char str1[10] = "fresh"; char str2[10] = "refresh"; if (!memcmp(str1,str2, 5*sizeof(char))) printf("Elements in str1 and str2 are same.\n"); else printf("Elements in str1 and str2 are not same.\n"); return 0; }

Output:

Elements in str1 and str2 are not same.

Example program for memicmp() function in C:

• memicmp( ) function is used to compare specified number of characters from two buffers regardless of the case of the characters.

• If we use memcmp() function instead of memicmp, the output of the below program will be “Elements in str1 and str2 are not same”.

#include <stdio.h> #include <string.h> int main() { // define two identical arrays char str1[10] = "fresh"; char str2[10] = "FRESH"; if (!memicmp(str1,str2, 5*sizeof(char))) printf("Elements in str1 and str2 are same.\n"); else printf("Elements in str1 and str2 are not same.\n"); return 0; }

Output:

Elements in str1 and str2 are same.

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Example program for memchr() function in C:

• memchr( ) function is used to locate the first occurrence of the character in the specified string.

#include <stdio.h> #include <string.h> int main () { char *ptr; char string[] = "fresh2refresh"; ptr = (char *) memchr (string, 'h', strlen(string)); if (ptr != NULL) printf ("character 'h' is found at " \ "position %d.\n", ptr-string+1); else printf ("character 'h' is not found.\n"); return 0; }

Output:

character ‘h’ is found at position 5.

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C – Time related functions

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Time functions in C are used to interact with system time routine and formatted time outputs are displayed. Example programs for the time functions are given below.

S.no Function Description 1 setdate() This function used to modify the system date 2 getdate() This function is used to get the CPU time 3 clock() This function is used to get current system time 4 time() This function is used to get current system time as structure 5 difftime() This function is used to get the difference between two given times 6 strftime() This function is used to modify the actual time format 7 mktime() This function interprets tm structure as calendar time 8 localtime() This function shares the tm structure that contains date and time informations 9 gmtime() This function shares the tm structure that contains date and time informations

10 ctime() This function is used to return string that contains date and time informations 11 asctime() Tm structure contents are interpreted by this function as calendar time. This time is converted into

string.

Example program for setdate() function in C:

This function is used to modify the system date. Please note that other C compilers may not support this setdate() function except turbo C.

#include<stdio.h> #include<dos.h> #include<conio.h> int main() { struct date dt; printf("Enter new date in the format(day month year)"); scanf("%d%d%d",&dt.da_day,&dt.da_mon,&dt.da_year); setdate(&dt); printf("Now, current system date is %d-%d-%d\n" ,dt.da_day,dt.da_mon,dt.da_year); return 0; }

Output:

Enter new date in the format (day month year) 01 12 2012 Now, current system date is 01-12-2012

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Example program for getdate() function in C:

This function is used to get the CPU time. Please note that other C compilers may not support this getdate() function except turbo C.

#include<stdio.h> #include<dos.h> int main() { struct date dt; getdate(&dt); printf("Operating system's current date is %d-%d-%d\n" ,dt.da_day,dt.da_mon,dt.da_year); return 0; }

Output:

Operating system’s current date is 12-01-2012

Example program for clock() function in C:

This function is used to get current system time

#include <stdio.h> #include <time.h> #include <math.h> int main() { int i; clock_t CPU_time_1 = clock(); printf("CPU start time is : %d \n", CPU_time_1); for(i = 0; i < 150000000; i++); clock_t CPU_time_2 = clock(); printf("CPU end time is : %d", CPU_time_2); }

Output:

CPU start time is : 0 CPU end time is : 380000

Example program for time() function in C:

This function is used to get current system time as structure

#include <stdio.h> #include <time.h> int main () { time_t seconds; seconds = time (NULL);

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printf ("Number of hours since 1970 Jan 1st " \ "is %ld \n", seconds/3600); return 0; }

Output:

Number of hours since 1970 Jan 1st is 374528

Example program for difftime() function in C:

This function is used to get the difference between two given times

#include <stdio.h> #include <time.h> int main() { time_t begin,end; long i; begin= time(NULL); for(i = 0; i < 150000000; i++); end = time(NULL); printf("for loop used %f seconds to complete the " \ "execution\n", difftime(end, begin)); return 0; }

Output:

for loop used 15.000000 seconds to complete the execution

Example program for strftime(), asctime() and localtime() in C:

• strftime() - This function is used to modify the actual time format. • asctime() – tm structure contents are interpreted by asctime() function as calendar time. This time is

converted into string. • localtime() – This function shares the tm structure that contains date and time informations.

#include <stdio.h> #include <time.h> #define LEN 150 int main () { char buf[LEN]; time_t curtime; struct tm *loc_time; //Getting current time of system curtime = time (NULL); // Converting current time to local time loc_time = localtime (&curtime); // Displaying date and time in standard format printf("%s", asctime (loc_time));

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strftime (buf, LEN, "Today is %A, %b %d.\n", loc_time); fputs (buf, stdout); strftime (buf, LEN, "Time is %I:%M %p.\n", loc_time); fputs (buf, stdout); return 0; }

Output:

Sat Sep 22 01:15:03 2012 Today is Saturday, Sep 22. Time is 01:15 AM.

Example program for mktime() and ctime() functions in C:

• mktime() function interprets tm structure as calendar time. • ctime() function is used to return string that contains date and time informations.

#include <stdio.h> #include <time.h> int main() { struct tm strtime; time_t timeoftheday; strtime.tm_year = 2008-1900; strtime.tm_mon = 1; strtime.tm_mday = 4; strtime.tm_hour = 02; strtime.tm_min = 30; strtime.tm_sec = 38; strtime.tm_isdst = 0; timeoftheday = mktime(&strtime); printf(ctime(&timeoftheday)); return 0; }

Output:

Mon Feb 4 02:30:38 2008

Example program for gmtime() function in C:

This function shares the tm structure that contains date and time informations.

#include <stdio.h> #include <time.h> int main() {

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time_t orig_format; time(&orig_format); printf ("Universal Time is %s", asctime(gmtime(&orig_format))); return 0; }

Output:

Universal Time is Sat Sep 22 08:11:40 2012

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C – Dynamic memory allocation

Dynamic memory allocation in C:

The process of allocating memory during program execution is called dynamic memory allocation.

Dynamic memory allocation functions in C:

C language offers 4 dynamic memory allocation functions. They are,

1. malloc() 2. calloc() 3. realloc() 4. free()

S.no Function Syntax 1 malloc () malloc (number *sizeof(int)); 2 calloc () calloc (number, sizeof(int)); 3 realloc () realloc (pointer_name, number * sizeof(int));4 free () free (pointer_name);

1. malloc() function in C:

• malloc () function is used to allocate space in memory during the execution of the program. • malloc () does not initialize the memory allocated during execution. It carries garbage value. • malloc () function returns null pointer if it couldn’t able to allocate requested amount of memory.

Example program for malloc() function in C:

#include <stdio.h> #include <string.h> #include <stdlib.h> int main() { char *mem_allocation; /* memory is allocated dynamically */ mem_allocation = malloc( 20 * sizeof(char) ); if( mem_allocation== NULL ) { printf("Couldn't able to allocate requested memory\n"); } else { strcpy( mem_allocation,"fresh2refresh.com"); } printf("Dynamically allocated memory content : " \ "%s\n", mem_allocation ); free(mem_allocation); }

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Output:

Dynamically allocated memory content : fresh2refresh.com

2. calloc() function in C:

• calloc () function is also like malloc () function. But calloc () initializes the allocated memory to zero. But, malloc() doesn’t.

Example program for calloc() function in C:

#include <stdio.h> #include <string.h> #include <stdlib.h> int main() { char *mem_allocation; /* memory is allocated dynamically */ mem_allocation = calloc( 20, sizeof(char) ); if( mem_allocation== NULL ) { printf("Couldn't able to allocate requested memory\n"); } else { strcpy( mem_allocation,"fresh2refresh.com"); } printf("Dynamically allocated memory content : " \ "%s\n", mem_allocation ); free(mem_allocation); }

Output:

Dynamically allocated memory content : fresh2refresh.com

3. realloc() function in C:

• realloc () function modifies the allocated memory size by malloc () and calloc () functions to new size. • If enough space doesn’t exist in memory of current block to extend, new block is allocated for the full size of

reallocation, then copies the existing data to new block and then frees the old block.

4. free() function in C:

• free () function frees the allocated memory by malloc (), calloc (), realloc () functions and returns the memory to the system.

Example program for realloc() and free() functions in C:

#include <stdio.h> #include <string.h> #include <stdlib.h> int main()

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{ char *mem_allocation; /* memory is allocated dynamically */ mem_allocation = malloc( 20 * sizeof(char) ); if( mem_allocation == NULL ) { printf("Couldn't able to allocate requested memory\n"); } else { strcpy( mem_allocation,"fresh2refresh.com"); } printf("Dynamically allocated memory content : " \ "%s\n", mem_allocation ); mem_allocation=realloc(mem_allocation,100*sizeof(char)); if( mem_allocation == NULL ) { printf("Couldn't able to allocate requested memory\n"); } else { strcpy( mem_allocation,"space is extended upto " \ "100 characters"); } printf("Resized memory : %s\n", mem_allocation ); free(mem_allocation); }

Output: Dynamically allocated memory content : fresh2refresh.comResized memory : space is extended upto 100 characters

Difference between static memory allocation and dynamic memory allocation in C:

S.no Static memory allocation Dynamic memory allocation 1 In static memory allocation, memory is allocated while writing

the C program. Actually, user requested memory will be allocated at compile time.

In dynamic memory allocation, memory is allocated while executing the program. That means at run time.

2 Memory size can’t be modified while execution. Example: array

Memory size can be modified while execution. Example: Linked list

Difference between malloc() and calloc() functions in C:

S.no malloc() calloc() 1 It allocates only single block of requested memory It allocates multiple blocks of requested memory 2 int *ptr;ptr = malloc( 20 * sizeof(int) );For the above,

20*4 bytes of memory only allocated in one block. Total = 80 bytes

int *ptr;Ptr = calloc( 20, 20 * sizeof(int) );For the above, 20 blocks of memory will be created and each contains 20*4 bytes of memory. Total = 1600 bytes

3 malloc () doesn’t initializes the allocated memory. It contains garbage values

calloc () initializes the allocated memory to zero

4 type cast must be done since this function returns void pointer int *ptr;ptr = (int*)malloc(sizeof(int)*20 );

Same as malloc () function int *ptr;ptr = (int*)calloc( 20, 20 * sizeof(int) );

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C – Type Casting functions

• Typecasting concept in C language is used to modify a variable from one date type to another data type. New data type should be mentioned before the variable name or value in brackets which to be typecast.

C type casting example program:

• In the below C program, 7/5 alone will produce integer value as 1. • So, type cast is done before division to retain float value (1.4).

#include <stdio.h> int main () { float x; x = (float) 7/5; printf("%f",x); }

Output:

1.400000

Note:

• It is best practice to convert lower data type to higher data type to avoid data loss. • Data will be truncated when higher data type is converted to lower. For example, if float is converted to int,

data which is present after decimal point will be lost.

Inbuilt typecast functions in C:

• There are many inbuilt typecasting functions available in C language which performs data type conversion from one type to another.

• Click on each function name below for description and example programs.

S.no Typecast function Description 1 atof() Converts string to float 2 atoi() Converts string to int 3 atol() Converts string to long 4 itoa() Converts int to string 5 ltoa() Converts long to string

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C – Miscellaneous functions

Descriptions and example programs for C environment functions such as getenv(), setenv(), putenv() and other functions perror(), random() and delay() are given below.

S.no Function Description

1 getenv() This function gets the current value of the environment variable

2 setenv() This function sets the value for environment variable

3 putenv() This function modifies the value for environment variable

4 perror() Displays most recent error that happened during library function call

5 rand() Returns random integer number range from 0 to at least 32767

6 delay() Suspends the execution of the program for particular time

Example program for getenv() function in C:

• This function gets the current value of the environment variable. • Let us assume that environment variable DIR is assigned to “/usr/bin/test/”. Below program will show you

how to get this value using getenv() function.

#include <stdio.h> #include <stdlib.h> int main() { printf("Directory = %s\n", getenv("DIR")); return 0; }

Output:

/usr/bin/test/

Example program for setenv() function in C:

• This function sets the value for environment variable. • Let us assume that environment variable “FILE” is to be assigned “/usr/bin/example.c”. Below program will

show you how to set this value using setenv() function.

#include <stdio.h> #include <stdlib.h> int main() { setenv("FILE","/usr/bin/example.c",50); printf("File = %s\n", getenv("FILE")); return 0; }

Output:

File = /usr/bin/example.c

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Example program for putenv() function in C:

• This function modifies the value of environment variable. • Below example program shows that how to modify an existing environment variable value.

#include <stdio.h> #include <stdlib.h> int main() { setenv("DIR","/usr/bin/example/",50); printf("Directory name before modifying = " \ "%s\n", getenv("DIR")); putenv("DIR=/usr/home/"); printf("Directory name after modifying = " \ "%s\n", getenv("DIR")); return 0; }

Output:

Directory name before modifying = /usr/bin/example/Directory name after modifying = /usr/home/

Example program for perror() function in C:

This function displays most recent error that happened during library function call.

#include <stdio.h> #include <errno.h> #include <stdlib.h> int main() { FILE *fp; char filename[40] = "test.txt"; /* Let us consider test.txt not available */ fp = f open(filename, "r"); if(fp == NULL) { perror("File not found"); printf("errno : %d.\n", errno); return 1; } printf("File is found and opened for reading"); fclose(fp); return 0; }

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Output:

errno : 22. File not found: No such file or directory

Example program for rand() function in C:

This function returns the random integer numbers range from 0 upto 32767

#include<stdio.h> #include<stdlib.h> #include<time.h> int main () { printf ("1st random number : %d\n", rand() % 100); printf ("2nd random number : %d\n", rand() % 100); printf ("3rd random number: %d\n", rand()); return 0; } Output: 1st random number : 83 2nd random number : 86 3rd random number: 16816927 Example program for delay() function in C: This function suspends the execution of the program for particular time. #include<stdio.h> #include<stdlib.h> int main () { printf("Suspends the execution of the program " \ "for particular time"); delay(5000); // 5000 mille seconds return 0; }

Output:

Suspends the execution of the program for particular time

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C – Structure

C Structure is a collection of different data types which are grouped together and each element in a C structure is called member.

• If you want to access structure members in C, structure variable should be declared. • Many structure variables can be declared for same structure and memory will be allocated for each

separately. • It is a best practice to initialize a structure to null while declaring, if we don’t assign any values to structure

members.

Difference between C variable, C array and C structure:

• A normal C variable can hold only one data of one data type at a time. • An array can hold group of data of same data type. • A structure can hold group of data of different data types • Data types can be int, char, float, double and long double etc.

Datatype C variable C array C structure Syntax Example Syntax Example Syntax Example

int int a a = 20 int a[3] a[0] = 10 a[1] = 20 a[2] = 30 a[3] = ‘\0′

struct student{ int a; char b[10]; }

a = 10 b = “Hello”

char char b b=’Z’ char b[10] b=”Hello”

Below table explains following concepts in C structure.

Type Using normal variable Using pointer variabe Syntax struct tag_name

{ data type var_name1; data type var_name2; data type var_name3; };

struct tag_name { data type var_name1; data type var_name2; data type var_name3; };

Example struct student { int mark; char name[10]; float average; };

struct student { int mark; char name[10]; float average; };

Declaring structure variable struct student report; struct student *report, rep; Initializing structure variable struct student report = {100, “Mani”, 99.5}; struct student rep = {100, “Mani”, 99.5};

report = &rep; Accessing structure members report.mark

report.name report.average

report -> mark report -> name report -> average

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Example program for C structure:

This program is used to store and access “id, name and percentage” for one student. We can also store and access these data for many students using array of structures. You can check “C – Array of Structures“ to know how to store and access these data for many students.

#include <stdio.h> #include <string.h> struct student { int id; char name[20]; float percentage; }; int main() { struct student record = {0}; //Initializing to null record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage); return 0; }

Output:

Id is: 1 Name is: Raju Percentage is: 86.500000

Example program – Another way of declaring C structure:

In this program, structure variable “record” is declared while declaring structure itself. In above structure example program, structure variable “struct student record” is declared inside main function which is after declaring structure.

#include <stdio.h> #include <string.h> struct student { int id; char name[20]; float percentage; } record; int main() { record.id=1;

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strcpy(record.name, "Raju"); record.percentage = 86.5; printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage); return 0; }

Output:


C structure declaration in separate header file:

In above structure programs, C structure is declared in main source file. Instead of declaring C structure in main source file, we can have this structure declaration in another file called “header file” and we can include that header file in main source file as shown below.

Header file name – structure.h

Before compiling and executing below C program, create a file named “structure.h” and declare the below structure.

struct student { int id; char name[20]; float percentage; } record;

Main file name – structure.c:

In this program, above created header file is included in “structure.c” source file as #include “Structure.h”. So, the structure declared in “structure.h” file can be used in “structure.c” source file.

// File name - structure.c #include <stdio.h> #include <string.h> #include "structure.h" /* header file where C structure is declared */ int main() { record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage); return 0; }

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Output:


Uses of C structures:

1. C Structures can be used to store huge data. Structures act as a database. 2. C Structures can be used to send data to the printer. 3. C Structures can interact with keyboard and mouse to store the data. 4. C Structures can be used in drawing and floppy formatting. 5. C Structures can be used to clear output screen contents. 6. C Structures can be used to check computer’s memory size etc.

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C – Array of Structures

As you know, C Structure is collection of different datatypes ( variables ) which are grouped together. Whereas, array of structures is nothing but collection of structures. This is also called as structure array in C.

Example program for array of structures in C:

This program is used to store and access “id, name and percentage” for 3 students. Structure array is used in this program to store and display records for many students. You can store “n” number of students record by declaring structure variable as ‘struct student record[n]“, where n can be 1000 or 5000 etc.

#include <stdio.h> #include <string.h> struct student { int id; char name[30]; float percentage; }; int main() { int i; struct student record[2]; // 1st student's record record[0].id=1; strcpy(record[0].name, "Raju"); record[0].percentage = 86.5; // 2nd student's record record[1].id=2; strcpy(record[1].name, "Surendren"); record[1].percentage = 90.5; // 3rd student's record record[2].id=3; strcpy(record[2].name, "Thiyagu"); record[2].percentage = 81.5; for(i=0; i<3; i++) { printf(" Records of STUDENT : %d \n", i+1); printf(" Id is: %d \n", record[i].id); printf(" Name is: %s \n", record[i].name); printf(" Percentage is: %f\n\n",record[i].percentage); } return 0; }

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Output:

Records of STUDENT : 1 Id is: 1 Name is: Raju Percentage is: 86.500000 Records of STUDENT : 2 Id is: 2 Name is: Surendren Percentage is: 90.500000 Records of STUDENT : 3 Id is: 3 Name is: Thiyagu Percentage is: 81.500000

Example program for declaring many structure variable in C:

In this program, two structure variables “record1″ and “record2″ are declared for same structure and different values are assigned for both structure variables. Separate memory is allocated for both structure variables to store the data.

#include <stdio.h> #include <string.h> struct student { int id; char name[30]; float percentage; }; int main() { int i; struct student record1 = {1, "Raju", 90.5}; struct student record2 = {2, "Mani", 93.5}; printf("Records of STUDENT1: \n"); printf(" Id is: %d \n", record1.id); printf(" Name is: %s \n", record1.name); printf(" Percentage is: %f \n\n", record1.percentage); printf("Records of STUDENT2: \n"); printf(" Id is: %d \n", record2.id); printf(" Name is: %s \n", record2.name); printf(" Percentage is: %f \n\n", record2.percentage); return 0; }

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Output:

Records of STUDENT1: Id is: 1 Name is: Raju Percentage is: 90.500000 Records of STUDENT2: Id is: 2 Name is: Mani Percentage is: 93.500000

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C – Passing struct to function

• A structure can be passed to any function from main function or from any sub function. • Structure definition will be available within the function only. • It won’t be available to other functions unless it is passed to those functions by value or by

address(reference). • Else, we have to declare structure variable as global variable. That means, structure variable should be

declared outside the main function. So, this structure will be visible to all the functions in a C program.

Passing structure to function in C:

It can be done in below 3 ways.

1. Passing structure to a function by value 2. Passing structure to a function by address(reference) 3. No need to pass a structure – Declare structure variable as global

Example program – passing structure to function in C by value:

In this program, the whole structure is passed to another function by value. It means the whole structure is passed to another function with all members and their values. So, this structure can be accessed from called function. This concept is very useful while writing very big programs in C.

#include <stdio.h> #include <string.h> struct student { int id; char name[20]; float percentage; }; void func(struct student record); int main() { struct student record; record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; func(record); return 0; } void func(struct student record) { printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage);

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}

Output:


Example program – Passing structure to function in C by address:

In this program, the whole structure is passed to another function by address. It means only the address of the structure is passed to another function. The whole structure is not passed to another function with all members and their values. So, this structure can be accessed from called function by its address.

#include <stdio.h> #include <string.h> struct student { int id; char name[20]; float percentage; }; void func(struct student *record); int main() { struct student record; record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; func(&record); return 0; } void func(struct student *record) { printf(" Id is: %d \n", record->id); printf(" Name is: %s \n", record->name); printf(" Percentage is: %f \n", record->percentage); }

Output:


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Example program to declare a structure variable as global in C:

Structure variables also can be declared as global variables as we declare other variables in C. So, When a structure variable is declared as global, then it is visible to all the functions in a program. In this scenario, we don’t need to pass the structure to any function separately.

#include <stdio.h> #include <string.h> struct student { int id; char name[20]; float percentage; }; struct student record; // Global declaration of structure void structure_demo(); int main() { record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; structure_demo(); return 0; } void structure_demo() { printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage); }

Output:


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C – Structure using Pointer

C structure can be accessed in 2 ways in a C program. They are,

1. Using normal structure variable 2. Using pointer variable

Dot(.) operator is used to access the data using normal structure variable and arrow (->) is used to access the data using pointer variable. You have learnt how to access structure data using normal variable in C – Structure topic. So, we are showing here how to access structure data using pointer variable in below C program.

Example program for C structure using pointer:

In this program, “record1″ is normal structure variable and “ptr” is pointer structure variable. As you know, Dot(.) operator is used to access the data using normal structure variable and arrow(->) is used to access data using pointer variable.

#include <stdio.h> #include <string.h> struct student { int id; char name[30]; float percentage; }; int main() { int i; struct student record1 = {1, "Raju", 90.5}; struct student *ptr; ptr = &record1; printf("Records of STUDENT1: \n"); printf(" Id is: %d \n", ptr->id); printf(" Name is: %s \n", ptr->name); printf(" Percentage is: %f \n\n", ptr->percentage); return 0; }

Output:

Records of STUDENT1: Id is: 1 Name is: Raju Percentage is: 90.500000

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Example program to copy a structure in C:

There are many methods to copy one structure to another structure in C.

1. We can copy using direct assignment of one structure to another structure or 2. we can use C inbuilt function “memcpy()” or 3. we can copy by individual structure members.

#include <stdio.h> #include <string.h> struct student { int id; char name[30]; float percentage; }; int main() { int i; struct student record1 = {1, "Raju", 90.5}; struct student record2, *record3, *ptr1, record4; printf("Records of STUDENT1 - record1 structure \n"); printf(" Id : %d \n Name : %s\n Percentage : %f\n", record1.id, record1.name, record1.percentage); // 1st method to copy whole structure to another structure record2=record1; printf("\nRecords of STUDENT1 - Direct copy from " \ "record1 \n"); printf(" Id : %d \n Name : %s\n Percentage : %f\n", record2.id, record2.name, record2.percentage); // 2nd method to copy using memcpy function ptr1 = &record1; memcpy(record3, ptr1, sizeof(record1)); printf("\nRecords of STUDENT1 - copied from record1 " \ "using memcpy \n"); printf(" Id : %d \n Name : %s\n Percentage : %f\n", record3->id, record3->name, record3->percentage); // 3rd method to copy by individual members printf("\nRecords of STUDENT1 - Copied individual " \ "members from record1 \n"); record4.id=record1.id; strcpy(record4.name, record1.name); record4.percentage = record1.percentage; printf(" Id : %d \n Name : %s\n Percentage : %f\n", record4.id, record4.name, record4.percentage); return 0; }

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Output:

Records of STUDENT1 - record1 structure Id : 1 Name : Raju Percentage : 90.500000 Records of STUDENT1 – Direct copy from record1 Id : 1 Name : Raju Percentage : 90.500000 Records of STUDENT1 – copied from record1 using memcpy Id : 1 Name : Raju Percentage : 90.500000 Records of STUDENT1 – Copied individual members from record1Id : 1 Name : Raju Percentage : 90.500000

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C – Nested Structure

Nested structure in C is nothing but structure within structure. One structure can be declared inside other structure as we declare structure members inside a structure. The structure variables can be a normal structure variable or a pointer variable to access the data. You can learn below concepts in this section.

1. Structure within structure in C using normal variable 2. Structure within structure in C using pointer variable

1. Structure within structure in C using normal variable:

This program explains how to use structure within structure in C using normal variable. “student_college_detail’ structure is declared inside “student_detail” structure in this program. Both structure variables are normal structure variables.

Please note that members of “student_college_detail” structure are accessed by 2 dot(.) operator and members of “student_detail” structure are accessed by single dot(.) operator.

#include <stdio.h> #include <string.h> struct student_college_detail { int college_id; char college_name[50]; }; struct student_detail { int id; char name[20]; float percentage; // structure within structure struct student_college_detail clg_data; }stu_data; int main() { struct student_detail stu_data = {1, "Raju", 90.5, 71145, "Anna University"}; printf(" Id is: %d \n", stu_data.id); printf(" Name is: %s \n", stu_data.name); printf(" Percentage is: %f \n\n", stu_data.percentage); printf(" College Id is: %d \n", stu_data.clg_data.college_id); printf(" College Name is: %s \n", stu_data.clg_data.college_name); return 0; }

Output:

Id is: 1

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Name is: Raju Percentage is: 90.500000 College Id is: 71145 College Name is: Anna University

Structure within structure in C using pointer variable:

This program explains how to use structure within structure in C using pointer variable. “student_college_detail’ structure is declared inside “student_detail” structure in this program. one normal structure variable and one pointer structure variable is used in this program.

Please note that combination of .(dot) and ->(arrow) operators are used to access the structure member which is declared inside the structure.

#include <stdio.h> #include <string.h> struct student_college_detail { int college_id; char college_name[50]; }; struct student_detail { int id; char name[20]; float percentage; // structure within structure struct student_college_detail clg_data; }stu_data, *stu_data_ptr; int main() { struct student_detail stu_data = {1, "Raju", 90.5, 71145, "Anna University"}; stu_data_ptr = &stu_data; printf(" Id is: %d \n", stu_data_ptr->id); printf(" Name is: %s \n", stu_data_ptr->name); printf(" Percentage is: %f \n\n", stu_data_ptr->percentage); printf(" College Id is: %d \n", stu_data_ptr->clg_data.college_id); printf(" College Name is: %s \n", stu_data_ptr->clg_data.college_name); return 0; } Output: Id is: 1 Name is: Raju Percentage is: 90.500000

College Id is: 71145 College Name is: Anna University

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C – Struct memory allocation

Do you know how memory is allocated for structure members in C?. You can learn below concepts of C in this topic.

1. how structure members are stored in memory? 2. What is structure padding? 3. How to avoid structure padding?

1. How structure members are stored in memory?

Always, contiguous(adjacent) memory locations are used to store structure members in memory. Consider below example to understand how memory is allocated for structures.

Example program for memory allocation in C structure:

#include <stdio.h> #include <string.h> struct student { int id1; int id2; char a; char b; float percentage; }; int main() { int i; struct student record1 = {1, 2, 'A', 'B', 90.5}; printf("size of structure in bytes : %d\n", sizeof(record1)); printf("\nAddress of id1 = %u", &record1.id1 ); printf("\nAddress of id2 = %u", &record1.id2 ); printf("\nAddress of a = %u", &record1.a ); printf("\nAddress of b = %u", &record1.b ); printf("\nAddress of percentage = %u",&record1.percentage); return 0; }

Output:

size of structure in bytes : 16 Address of id1 = 675376768 Address of id2 = 675376772 Address of a = 675376776 Address of b = 675376777 Address of percentage = 675376780

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There are 5 members declared for structure in above program. In 32 bit compiler, 4 bytes of memory is occupied by int datatype. 1 byte of memory is occupied by char datatype and 4 bytes of memory is occupied by float datatype.

Please refer below table to know from where to where memory is allocated for each datatype in contiguous (adjacent) location in memory.

Datatype Memory allocation in C (32 bit compiler) From Address To Address Total bytes

int id1 675376768 675376771 4 int id2 675376772 675376775 4char a 675376776 1 char b 675376777 1

Addresses 675376778 and 675376779 are left empty (Do you know why? Please see Structure padding topic below)

2

float percentage 675376780 675376783 4

The pictorial representation of above structure memory allocation is given below. This diagram will help you to understand the memory allocation concept in C very easily.

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C – Structure Padding

• In order to align the data in memory, one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding.

• Architecture of a computer processor is such a way that it can read 1 word (4 byte in 32 bit processor) from memory at a time.

• To make use of this advantage of processor, data are always aligned as 4 bytes package which leads to insert empty addresses between other member’s address.

• Because of this structure padding concept in C, size of the structure is always not same as what we think.

For example, please consider below structure that has 5 members. .. struct student { int id1; int id2; char a; char b; float percentage; }; ..

• As per C concepts, int and float datatypes occupy 4 bytes each and char datatype occupies 1 byte for 32 bit processor. So, only 14 bytes (4+4+1+1+4) should be allocated for above structure.

• But, this is wrong. Do you know why?

• Architecture of a computer processor is such a way that it can read 1 word from memory at a time. • 1 word is equal to 4 bytes for 32 bit processor and 8 bytes for 64 bit processor. So, 32 bit processor always

reads 4 bytes at a time and 64 bit processor always reads 8 bytes at a time. • This concept is very useful to increase the processor speed. • To make use of this advantage, memory is arranged as a group of 4 bytes in 32 bit processor and 8 bytes in

64 bit processor.

• Below C program is compiled and executed in 32 bit compiler. Please check memory allocated for structure1 and structure2 in below program.

Example program for structure padding in C:

#include <stdio.h> #include <string.h> /* Below structure1 and structure2 are same. They differ only in member's allignment */ struct structure1 { int id1; int id2; char name; char c; float percentage;

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}; struct structure2 { int id1; char name; int id2; char c; float percentage; }; int main() { struct structure1 a; struct structure2 b; printf("size of structure1 in bytes : %d\n", sizeof(a)); printf ( "\n Address of id1 = %u", &a.id1 ); printf ( "\n Address of id2 = %u", &a.id2 ); printf ( "\n Address of name = %u", &a.name ); printf ( "\n Address of c = %u", &a.c ); printf ( "\n Address of percentage = %u", &a.percentage ); printf(" \n\nsize of structure2 in bytes : %d\n", sizeof(b)); printf ( "\n Address of id1 = %u", &b.id1 ); printf ( "\n Address of name = %u", &b.name ); printf ( "\n Address of id2 = %u", &b.id2 ); printf ( "\n Address of c = %u", &b.c ); printf ( "\n Address of percentage = %u", &b.percentage ); getchar(); return 0; }

Output:

size of structure1 in bytes : 16 Address of id1 = 1297339856 Address of id2 = 1297339860 Address of name = 1297339864 Address of c = 1297339865 Address of percentage = 1297339868 size of structure2 in bytes : 20 Address of id1 = 1297339824 Address of name = 1297339828 Address of id2 = 1297339832 Address of c = 1297339836 Address of percentage = 1297339840

Structure padding analysis for above C program:

Memory allocation for structure1:

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• In above program, memory for structure1 is allocated sequentially for first 4 members. • Whereas, memory for 5th member “percentage” is not allocated immediate next to the end of member “c”. • There are only 2 bytes remaining in the package of 4 bytes after memory allocated to member “c”. • Range of this 4 byte package is from 1297339864 to 1297339867.

• Addresses 1297339864 and 1297339865 are used for members “name and c”. Addresses 1297339866 and 1297339867 only is available in this package.

• But, member “percentage” is datatype of float and requires 4 bytes. It can’t be stored in the same memory package as it requires 4 bytes. Only 2 bytes are free in that package.

• So, next 4 byte of memory package is chosen to store percentage data which is from 1297339868 to 1297339871.

• Because of this, memory 1297339866 and 1297339867 are not used by the program and those 2 bytes are left empty.

• So, size of structure1 is 16 bytes which is 2 bytes extra than what we think. Because, 2 bytes are left empty.

Memory allocation for structure2:

• Memory for structure2 is also allocated as same as above concept. Please note that structure1 and structure2 are same. But, they differ only in the order of the members declared inside the structure.

• 4 bytes of memory is allocated for 1st structure member “id1″ which occupies whole 4 byte of memory package.

• Then, 2nd structure member “name” occupies only 1 byte of memory in next 4 byte package and remaining 3 bytes are left empty. Because, 3rd structure member “id2″ of datatype integer requires whole 4 byte of memory in the package. But, this is not possible as only 3 bytes available in the package.

• So, next whole 4 byte package is used for structure member “id2″. • Again, 4th structure member “c” occupies only 1 byte of memory in next 4 byte package and remaining 3

bytes are left empty.

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• Because, 5th structure member “percentage” of datatype float requires whole 4 byte of memory in the package.

• But, this is also not possible as only 3 bytes available in the package. So, next whole 4 byte package is used for structure member “percentage”.

• So, size of structure2 is 20 bytes which is 6 bytes extra than what we think. Because, 6 bytes are left empty.

How to avoid structure padding in C?

• #pragma pack ( 1 ) directive can be used for arranging memory for structure members very next to the end of other structure members.

• VC++ supports this feature. But, some compilers such as Turbo C/C++ does not support this feature. • Please check the below program where there will be no addresses (bytes) left empty because of structure

padding.

Example program to avoid structure padding in C:

#include <stdio.h> #include <string.h> /* Below structure1 and structure2 are same. They differ only in member's allignment */ #pragma pack(1) struct structure1 { int id1; int id2; char name; char c; float percentage; }; struct structure2 { int id1; char name; int id2; char c; float percentage; }; int main() { struct structure1 a; struct structure2 b; printf("size of structure1 in bytes : %d\n", sizeof(a)); printf ( "\n Address of id1 = %u", &a.id1 ); printf ( "\n Address of id2 = %u", &a.id2 ); printf ( "\n Address of name = %u", &a.name ); printf ( "\n Address of c = %u", &a.c ); printf ( "\n Address of percentage = %u", &a.percentage ); printf(" \n\nsize of structure2 in bytes : %d\n", sizeof(b)); printf ( "\n Address of id1 = %u", &b.id1 ); printf ( "\n Address of name = %u", &b.name ); printf ( "\n Address of id2 = %u", &b.id2 );

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printf ( "\n Address of c = %u", &b.c ); printf ( "\n Address of percentage = %u", &b.percentage ); getchar(); return 0; }

Output:

size of structure1 in bytes : 14 Address of id1 = 3438103088 Address of id2 = 3438103092 Address of name = 3438103096 Address of c = 3438103097 Address of percentage = 3438103098 size of structure2 in bytes : 14 Address of id1 = 3438103072 Address of name = 3438103076 Address of id2 = 3438103077 Address of c = 3438103081 Address of percentage = 3438103082

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C – Typedef

• Typedef is a keyword that is used to give a new symbolic name for the existing name in a C program. This is same like defining alias for the commands.

• Consider the below structure.

struct student { int mark [2]; char name [10]; float average; }

• Variable for the above structure can be declared in two ways.

1st way :

struct student record; /* for normal variable */ struct student *record; /* for pointer variable */

2nd way :

typedef struct student status;

• When we use “typedef” keyword before struct <tag_name> like above, after that we can simply use type definition “status” in the C program to declare structure variable.

• Now, structure variable declaration will be, “status record”. • This is equal to “struct student record”. Type definition for “struct student” is status. i.e. status = “struct

student”

An alternative way for structure declaration using typedef in C:

typedef struct student { int mark [2]; char name [10]; float average; } status;

• To declare structure variable, we can use the below statements.

status record1; /* record 1 is structure variable */ status record2; /* record 2 is structure variable */

Example program for C typedef:

// Structure using typedef: #include <stdio.h> #include <string.h>

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typedef struct student { int id; char name[20]; float percentage; } status; int main() { status record; record.id=1; strcpy(record.name, "Raju"); record.percentage = 86.5; printf(" Id is: %d \n", record.id); printf(" Name is: %s \n", record.name); printf(" Percentage is: %f \n", record.percentage); return 0; }

Output:


• Typedef can be used to simplify the real commands as per our need. • For example, consider below statement.

typedef long long int LLI;

• In above statement, LLI is the type definition for the real C command “long long int”. We can use type definition LLI instead of using full command “long long int” in a C program once it is defined.

Another example program for C typedef:

#include <stdio.h> #include <limits.h> int main() { typedef long long int LLI; printf("Storage size for long long int data " \ "type : %ld \n", sizeof(LLI)); return 0; }

Output:

Storage size for long long int data type : 8

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C – Union

C Union is also like structure, i.e. collection of different data types which are grouped together. Each element in a union is called member.

• Union and structure in C are same in concepts, except allocating memory for their members. • Structure allocates storage space for all its members separately. • Whereas, Union allocates one common storage space for all its members

• We can access only one member of union at a time. We can’t access all member values at the same time in union. But, structure can access all member values at the same time. This is because, Union allocates one common storage space for all its members. Where as Structure allocates storage space for all its members separately.

• Many union variables can be created in a program and memory will be allocated for each union variable separately.

• Below table will help you how to form a C union, declare a union, initializing and accessing the members of the union.

Type Using normal variable Using pointer variable Syntax union tag_name

{ data type var_name1; data type var_name2; data type var_name3; };

union tag_name { data type var_name1; data type var_name2; data type var_name3; };

Example union student { int mark; char name[10]; float average; };

union student { int mark; char name[10]; float average; };

Declaring union variable union student report; union student *report, rep; Initializing union variable union student report = {100, “Mani”, 99.5}; union student rep = {100, “Mani”, 99.5};report = &rep;Accessing union members report.mark

report.name report.average

report -> mark report -> name report -> average

Example program for C union:

#include <stdio.h> #include <string.h> union student { char name[20]; char subject[20]; float percentage; };

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int main() { union student record1; union student record2; // assigning values to record1 union variable strcpy(record1.name, "Raju"); strcpy(record1.subject, "Maths"); record1.percentage = 86.50; printf("Union record1 values example\n"); printf(" Name : %s \n", record1.name); printf(" Subject : %s \n", record1.subject); printf(" Percentage : %f \n\n", record1.percentage); // assigning values to record2 union variable printf("Union record2 values example\n"); strcpy(record2.name, "Mani"); printf(" Name : %s \n", record2.name); strcpy(record2.subject, "Physics"); printf(" Subject : %s \n", record2.subject); record2.percentage = 99.50; printf(" Percentage : %f \n", record2.percentage); return 0; }

Output:

Union record1 values example Name : Subject : Percentage : 86.500000; Union record2 values example Name : Mani Subject : Physics Percentage : 99.500000

Explanation for above C union program:

There are 2 union variables declared in this program to understand the difference in accessing values of union members.

Record1 union variable:

• “Raju” is assigned to union member “record1.name” . The memory location name is “record1.name” and the value stored in this location is “Raju”.

• Then, “Maths” is assigned to union member “record1.subject”. Now, memory location name is changed to “record1.subject” with the value “Maths” (Union can hold only one member at a time).

• Then, “86.50” is assigned to union member “record1.percentage”. Now, memory location name is changed to “record1.percentage” with value “86.50”.

• Like this, name and value of union member is replaced every time on the common storage space. • So, we can always access only one union member for which value is assigned at last. We can’t access other

member values. • So, only “record1.percentage” value is displayed in output. “record1.name” and “record1.percentage” are

empty.

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Record2 union variable:

• If we want to access all member values using union, we have to access the member before assigning values to other members as shown in record2 union variable in this program.

• Each union members are accessed in record2 example immediately after assigning values to them. • If we don’t access them before assigning values to other member, member name and value will be over

written by other member as all members are using same memory. • We can’t access all members in union at same time but structure can do that.

Example program – Another way of declaring C union:

In this program, union variable “record” is declared while declaring union itself as shown in the below program.

#include <stdio.h> #include <string.h> union student { char name[20]; char subject[20]; float percentage; }record; int main() { strcpy(record.name, "Raju"); strcpy(record.subject, "Maths"); record.percentage = 86.50; printf(" Name : %s \n", record.name); printf(" Subject : %s \n", record.subject); printf(" Percentage : %f \n", record.percentage); return 0; }

Output:

Name : Subject : Percentage : 86.500000

Note:

We can access only one member of union at a time. We can’t access all member values at the same time in union. But, structure can access all member values at the same time. This is because, Union allocates one common storage space for all its members. Where as Structure allocates storage space for all its members separately.

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Difference between structure and union in C:

S.no C Structure C Union1 Structure allocates storage space for

all its members separately. Union allocates one common storage space for all its members. Union finds that which of its member needs high storage space over other members and allocates that much space

2 Structure occupies higher memory space.

Union occupies lower memory space over structure.

3 We can access all members of structure at a time.

We can access only one member of union at a time.

4 Structure example: struct student { int mark; char name[6]; double average; };

Union example: union student { int mark; char name[6]; double average; };

5 For above structure, memory allocation will be like below. int mark – 2B char name[6] – 6B double average – 8B Total memory allocation = 2+6+8 = 16 Bytes

For above union, only 8 bytes of memory will be allocated since double data type will occupy maximum space of memory over other data types. Total memory allocation = 8 Bytes

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C – Preprocessor directives

C Preprocessor directives:

• Before a C program is compiled in a compiler, source code is processed by a program called preprocessor. This process is called preprocessing.

• Commands used in preprocessor are called preprocessor directives and they begin with “#” symbol. • Below is the list of preprocessor directives that C language offers.

S.no Preprocessor Syntax Description 1 Macro #define This macro defines constant value and can be any of the basic data types. 2 Header file

inclusion #include <file_name> The source code of the file “file_name” is included in the main program at

the specified place 3 Conditional

compilation #ifdef, #endif, #if, #else, #ifndef

Set of commands are included or excluded in source program before compilation with respect to the condition

4 Other directives #undef, #pragma #undef is used to undefine a defined macro variable. #Pragma is used to call a function before and after main function in a C program

A program in C language involves into different processes. Below diagram will help you to understand all the processes that a C program comes across.

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Example program for #define, #include preprocessors in C:

• #define - This macro defines constant value and can be any of the basic data types. • #include <file_name> - The source code of the file “file_name” is included in the main C program where

“#include <file_name>” is mentioned.

#include <stdio.h> #define height 100 #define number 3.14 #define letter 'A' #define letter_sequence "ABC" #define backslash_char '\?' void main() { printf("value of height : %d \n", height ); printf("value of number : %f \n", number ); printf("value of letter : %c \n", letter ); printf("value of letter_sequence : %s \n", letter_sequence); printf("value of backslash_char : %c \n", backslash_char); }

Output:


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Example program for conditional compilation directives:

a) Example program for #ifdef, #else and #endif in C:

• “#ifdef” directive checks whether particular macro is defined or not. If it is defined, “If” clause statements are included in source file.

• Otherwise, “else” clause statements are included in source file for compilation and execution.

#include <stdio.h> #define RAJU 100 int main() { #ifdef RAJU printf("RAJU is defined. So, this line will be added in " \ "this C file\n"); #else printf("RAJU is not defined\n"); #endif return 0; }

Output:

RAJU is defined. So, this line will be added in this C file

b) Example program for #ifndef and #endif in C:

• #ifndef exactly acts as reverse as #ifdef directive. If particular macro is not defined, “If” clause statements are included in source file.

• Otherwise, else clause statements are included in source file for compilation and execution.

#include <stdio.h> #define RAJU 100 int main() { #ifndef SELVA { printf("SELVA is not defined. So, now we are going to " \ "define here\n"); #define SELVA 300 } #else printf("SELVA is already defined in the program”); #endif return 0; }

Output:

SELVA is not defined. So, now we are going to define here

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c) Example program for #if, #else and #endif in C:

• “If” clause statement is included in source file if given condition is true. • Otherwise, else clause statement is included in source file for compilation and execution.

#include <stdio.h> #define a 100 int main() { #if (a==100) printf("This line will be added in this C file since " \ "a \= 100\n"); #else printf("This line will be added in this C file since " \ "a is not equal to 100\n"); #endif return 0; }

Output:

This line will be added in this C file since a = 100

Example program for undef in C:

This directive undefines existing macro in the program.

#include <stdio.h> #define height 100 void main() { printf("First defined value for height : %d\n",height); #undef height // undefining variable #define height 600 // redefining the same for new value printf("value of height after undef \& redefine:%d",height); }

Output:

First defined value for height : 100 value of height after undef & redefine : 600

Example program for pragma in C:

Pragma is used to call a function before and after main function in a C program.

#include <stdio.h> void function1( ); void function2( ); #pragma startup function1 #pragma exit function2 int main( )

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{ printf ( "\n Now we are in main function" ) ; return 0; } void function1( ) { printf("\nFunction1 is called before main function call"); } void function2( ) { printf ( "\nFunction2 is called just before end of " \ "main function" ) ;" }

Output:

Function1 is called before main function call Now we are in main function Function2 is called just before end of main function

More on pragma directive in C:

S.no Pragma command description1 #Pragma startup

<function_name_1> This directive executes function named “function_name_1” before

2 #Pragma exit <function_name_2> This directive executes function named “function_name_2” just before termination of the program.

3 #pragma warn – rvl If function doesn’t return a value, then warnings are suppressed by this directive while compiling.

4 #pragma warn – par If function doesn’t use passed function parameter , then warnings are suppressed 5 #pragma warn – rch If a non reachable code is written inside a program, such warnings are suppressed by

this directive.

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Documents

IDFHERRN FRP HGXQHSDO LQIR # HGXQHSDOBLQIRedunepal.info/wp-content/uploads/2015/09/C-Prog.-Point...C printf() function: • printf() function is used to print the “character, string,